Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/461

Rh CLASSIFICATION.] REPTILES 443 Os quadratum suturally connected with the skull. The premaxillary bones are separate, and the rami of the lower jaw united in front by cartilage only. Form of the teeth variable ; they are not anchylosed to the bone. Two pairs of limbs are present, of which the hinder pair is the longer and larger, and generally ambulatory. The struc- ture of the pelvis and hind limbs partly Ornithic ; the pelvic bones are not coalesced with each other or with the sacrum ; the pubis enters into the formation of the ace- tabulum, and, the ilium is prolonged forwards in front of the acetabulum ; ischia united in a median ventral symphysis. The head of the femur is placed at a right angle to the condyles ; tibia with a procnemial crest, and a ridge for the fibula, which is complete. The proximal row of tarsals is formed by the astragalus and calcaneum only, and the former sometimes anchylosed with the tibia, thus forming the upper portion of the ankle-joint. 1 1 In the case of no other group of Reptiles lias knowledge, within the last few years, advanced so much as in that of Dinosaurians. It has supplied, on the part of the Reptilian type, the remarkable forms by which the chasm between living Birds and Reptiles is bridged over. Huxley's interpretation as to the affinities of these fossils, which was at first based on very imperfect materials, but on sufficient evidence to lead him to substitute the name of Ornithoscelida for the older one of Dinosauria, has been fully verified by the astonishing discoveries of most perfect remains at Beruissart in Belgium and in Jurassic formations of the United States. Compared with these recent dis- coveries, the materials upon which Von Meyer, Owen, Leidy, Hulke, and Sceley based their researches must appear very fragmentary. The specimens found in the Wealden of Bernissart were those of different species of fguanodon, the skeletons of which were almost complete, the bones being preserved in their natural position and connexion ; they have formed the subject of a series of memoirs by L. Dollo (Lull. Mus. R. d'Hist. Nat. Bdg., 1882-84). But these materials are far surpassed, as regards number and diversity of forms, by the discoveries in America, which have been made known by Cope and especially by 0. C. Marsh. All the Dinosaurian remains known at present belong to the Mesozoic age ; they appear first in the Triassic, but the majority from these formations are so fragmentary that their classification is subject to much uncertainty. Some at least of the celebrated three-toed foot- prints which were discovered some fifty years ago in the United States, and about the origin of which much uncertainty existed, are evidently those of Dinosaurs. In the Jurassic these Reptiles attained their greatest development, Marsh distinguishing among the fossils of this period four orders with numerous families. The largest exceeded any other land animal in size, and measured from 50 to 80 feet. Dinosaurs continued to the end of the Cretaceous period, and some genera became highly specialized, although none attained to the same large size as some of the Jurassic forms. The disproportion in length and strength between the fore and hind limbs clearly shows that the mode of progression differed widely from that of ordinary Reptiles, and was bipedal at least in some of the genera. In assuming an erect position, their long tail assisted them in balanc- ing the body. Some possessed a dermal armour, the scutes being sometimes produced into enormous spines ; others were provided with defensive weapons in the shape of spines attached to the fore-feet. The teeth vary exceedingly : in some of the carnivorous genera they are sharp, pointed, serrated, and recurved, in others flat, in others broad and molar-like ; in some the premaxillasare toothed, in others toothless and beak-like. Marsh compares the Dinosaurs, as regards diversity of form, with the Marsupials, and thinks that, like these latter, they should take the rank of a subclass rather than order. The following is an abstract of his latest classification of the proposed subclass Dino- sauria : Order 1. SAUROPODA. Feet plantigrade, ungulate ; five digits in manus and pes ; second row of tarsals and carpals unossified. Pubes united distally by cartilage ; no post-pubis. Anterior vertebrae opisthoccelian. Presacral vertebra hollow. Fore and hind limbs nearly equal ; limb bones solid. Sternal bones paired. Premaxillaries with teeth. Anterior nares at top of the skull. Herbivorous. Fam. a. Atlantosauridse. Ischia directed downwards, with ex- panded extremities meeting on median line. Anterior caudal verte- brae with lateral cavities. Genera: Atlanta saur us, Apatosaurus, Bronto- saurus. Gigantic Dinosaurians from Upper Jurassic deposits of Colorado, species of the first genus having attained to the enormous length of 80 feet. They are the least specialized forms of the subclass, and approach in some respects Mesozoic Crocodilians. Fam. b. Diplododdee. Ischia with a straight shaft, not expanded distally, directed downward and backward, with the ends meeting in the median line. Caudal vertebrae deeply excavated below. One genus: Order 4. ORNITHOSAURIA (extinct). Reptiles with the fore limb adapted to support a flying membrane, and Diplodocus, from the same formation as the preceding, with very weak dentition, limited to the fore part of the jaws. Fain. c. Morosauridas. Ischia slender, with twisted shaft, directed backward, and with the sides meeting in the median line. Anterior caudal vertebrae solid. Genera, occurring in Europe as well as America : Bothriospondylus, Celiosaurus, Chondrosteosaurus, Euca- vierotus, Ornithopsis, Pelorosaurus. Order 2. STEGOSAURIA. Feet plantigrade, ungulate ; five digits in manus and pes ; second row of carpals unossified. Post-pubis present. Fore limbs very small ; locomotion mainly on hind limbs. Vertebrae and limb bones solid. An osseous dermal armour. Herbivorous. Fam. a. Stegosauridas. Vertebrae biconcave. Ischia directed back- ward, with the sides meeting in the median line. Astragalus coalesced with tibia. Metatarsal short. Genera : Stegosaurus (Hypsirhophus), some 30 feet long, from the Jurassic beds of the Rocky Mountain region, well armed with enormous bucklers, some of which bore spines; Diracodon; Amosaurus, from British formations (Kimme- ridge Clay). Fam. 6. Scelidosauridaz. Astragalus not coalesced with tibia ; metatarsals elongate. Genera European : Scelidosaurus, from the Lias ; A canthopholis, from the Chalk ; Cratseomus ; Hyleeosaurus, from the Wealden ; and Polacanthus. Order 3. ORNITHOPODA. Feet digitigrade with five functional digits in manus and three or four in pes. Post-pubis present. Vertebras solid. Fore limbs small ; hind-limbs hollow. Premaxil- laries edentulous in front. Herbivorous. Fam. a. Hadrosauridse. Several series of teeth, forming with use a tessellated grinding surface. Anterior vertebrae opisthoccelous. Genera American: lladrosaurus, Agatliaumas {?), C'ionodon. Fam. b. Hypsilophodontidse. A single series of teeth. Four functional digits in pes. A single rhomboidal sternal ossification. Genus : Hypsilophodon, from the Wealden of the Isle of Wight. Fam. c. Iguanodontidse. A single row of teeth. Three functional digits in pes. Two symmetrical sternal ossifications. Two genera from Europe : lyuanodon and Vectisaurus ; and three comparatively small forms from the Dinosaurian deposits in North America : Camptonotus, Laosaurus, Nanosdurus. Of these Iguanodon is the one which was first discovered (1825), and of which skeletons have been obtained a3 complete as we can ever hope to see of these creatures. The remains occur in formations from the Kimmeridge Clay to the Upper Greensand, and have been referred to three species, varying in size from 10 to 35 feet in length. They most probably were aquatic in their habits, using their powerful tail as a propelling organ, like the Crocodiles ; but they differed from them in their mode of locomotion on shore, walking on their hind legs like a Struthious Bird. (See fig. 1.) Order 4. THEROPODA. Feet digitigrade ; digits with prehensile claws. Pubes distally coalesced. Vertebrae more or less cavernous. Fore limbs very small ; limb bones hollow. Premaxillaries with teeth. Carnivorous. This order, although on the whole comprising less gigantic fossils than the preceding, includes some very large forms which are believed to have preyed upon the weaker herbivorous members of Dinosaurians. Fam. a. Megalosauridse. Vertebras biconcave. Pubes slender and united distally. Astragalus with ascending process. Five digits in mauus and four in pes. Genera: Meyalosaurus (European) ; Allosaurus, Coelosaurus, Creosaurus, Dryptosaurus (L&laps) (American). Fam. b. Zanclodontidse. Vertebrae biconcave. Pubes broad, elongate plates with anterior margins united. Astragalus without ascending process. Five digits in manus and pes. Genera : Zanclodon, Teratosaurus (? ) from the European Trias. Fam. c. Amphisauridas. Vertebras biconcave. Pubes rod-like ; five digits in manus and three in pes. Genera : Amphisaurus (Mega- dactylus], Bathygnathus(^), Clepsysaurus(!) (American); Pal&osaurus and Thecodontosaurus, European Trias. Fam. d. Labrosauridae. Anterior vertebras opisthocoelous and cavernous. Metatarsals much elongated. Pubes slender, with anterior margins united. Genus : Labrosaurus, from America. Fam. e. (or suborder) Ccduria. Bones pneumatic or hollow. Anterior cervical vertebras opisthoccelous, the other biconcave. Metatarsals very long and slender. Genus : Ccelurus, from America. Very imperfectly known ; the remains indicate animals not larger than a wolf, and possibly of arboreal habits. Fam. /. (or suborder) Compsognatha. Anterior vertebras opis- thoccelous. Three functional digits in manus and pes. Ischia with a long symphysis in the median line. One genus : Compsognathus, from Solenhofen ; a small form, with long neck, lightly built head, strongly toothed jaws, small fore and very long hind limbs ; its femur was shorter than the tibia. Order 5. HALLOPODA. Feet digitigrade, unguiculate ; three digits in pes ; metatarsals much elongate ; calcaneum much produced backwards. Fore limbs very small ; vertebrae and limb bones hollow ; vertebrae biconcave. Genus : Hallqpus. Its pertinence to the Dino- sauriaus is doubtful. The hind feet were adapted for leaping.