Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/814

Rh 770 AMPHIBIA occurs in the Lias of Simbirsk. I n the earliest Mesozoic deposi ts the Trias, and in the later Palaeozoic the Permian and the Carboni ferous formations, Amphibia occur, sometimes in great abundance. In the Trias, they have been found in greatest numbers in Germany, while the Carboniferous formations have furnished the largest supply in the British Islands, Germany, and North America. It is inter esting to observe that the last-named region has recently yielded elongated apodal forms, allied to the Oplvidcrpeton of the Kilkenny coal measure*;. dBtiology of the Amphibia. In taking a general survey of the relations of the diiferent great divisions of the Amphibia, the most striking fact is their singular distinctness and isolation from one another. None of the Peromela present the slightest indication of an approximation towards the Anura or the Urodela. It may be suggested that the incompleteness of the jugal arch in Breviceps, Pipa, and Dactylethra; the absence or rudimentary condi tion of the palatine bones in Breviceps, Bombinator, and Alytes; the rudimentary condition of the tympanum, and the absence, or reduction to a rudiment, of the columella auris, in so many forms ; the presence of rudimentary ribs attached to some of the anterior vertebrae of Bombinator and Alytes; the presence of mandibular teeth in Hemiphractus and Grypiscus; and the peculiar spermatozoa of Bombinator, are so many indications of an approach towards the type of structure observed in the higher Urodela. But, without underestimating the force of these considerations, it must be admitted that they count for very little, when we take into consideration the fixity of the number of the vertebrae, and of the characters of the pelvis and of the limbs, in the Anura. It is to be regretted that nothing is known of the development of any of the Peromela; of any of the Urodela, except Salamandra, Triton, and Siredon (A7tiblystoma) ; and of more than a few of the Anura. Among the lower forms of this division, the development of Alytes and Pelobates has been studied thoroughly by Vogt 1 and Van Bambeke; 2 and the more advanced conditions of the tadpole of Dactylethra are known. So far as these observations go, however, they tend to show that the larvae of all the Anura possess the horny beak, which distinguishes them from those of the Urodela. If we assume, as the fundamental similarities between the different divisions of the Amphibia lead us to do, that they have resulted from the modification of some one primitive form, the problem, at present seemingly insoluble, presents itself, whether these difi er- ences in structure and habit of the larvas of the Urodela and Anura indicate that the caudate ancestor of the Anura was already different from the ancestor of the Urodela, or whether they result from modi fications which have taken place in the larvae of the Anura, since that group came into existence. In view of this problem, Siren possesses a particular interest. Its horny jaw-sheaths might be compared to those of the Anuran tad pole, and it might be regarded as shoving the way by which the Anurau became differentiated from the caudate original stock. But the horny sheaths in Siren rest directly upon the premaxillae and the dentaries, and not on labial cartilages ; and as to its habits of life, Siren appears to be eminently carnivorous (Dumeril et Bibron, Erpetologie Generale, i. 196). As has been already stated, no fossil remains of Peromela are known, but Urodela and Anura occur in some abundance, and, in certain cases, in an excellent state of preservation, as far back as the middle of the Tertiary epoch. Now, these fossils show that the Anurous and Urodelous types of organisation were, at that time, thoroughly differentiated from one another. Palceobatrachus, with its three vertebrae ankylosed into a sacrum, is, in fact, a singularly modified frog ; while among the Urodela, the Salamandrida, the Mcnopomida, and very possibly the Proteida, are severally represented. The young of the Miocene Anura were tadpoles so similar in form to those of the existing frogs and toads, that there is no reason to doubt their resembling them in other respects. There can be little question, then, that the Anurous and the Uro delous types must have been represented before the Tertiary epoch ; but here their history breaks off, no amphibian belonging to any living groups having been discovered in Mesozoic or older strata, as far as the Lias. From the Trias to the Carboniferous formations, inclusively, the fresh-water deposits abound in Amphibia. But all these, so far as we have any positive knowledge, are referable to the Labyrintho- dont type. No Labyrinthodont presents the slightest approxima tion towards the Anura; but elongated and apodal, aa well as sala- mandroid forms occur ; and in their cranial structure, no less than in the presence of scale-like dermal ossifications, they approach the Peromela. In regard to their possible relations with the Urodela, it is interesting to observe that in some Labyrinthodonts, at any 1 Yogt, Unt&stuhunyen uber die Enlwitkelungsgeschichte der Gflurts- helferkrute (Alytes obstetricans), 1842. - Van Bambeke, &quot; Recherches sur le developpement du Pelobate brim,&quot; Man. de I Acad. de Belgique, 1368. rate, 5 the manus has the five digits, one of which, at least, is lost in all the Urodela, and the pelvis appears to have had a distinct and completely ossified pubic element, which has also disappeared in all existing Amphibia, (Miall, Report, I.e.) The Labyrinthodonts present a few characters such as the paired supra-occipital ossifications and the complications of the folds of their teeth by which they approach the Ganoid fishes more than any other Vertebrata ; and it is worthy of notice that the lowest Labyrinthodonts, such as Archcgosaurus, present no approxima tion to the cranial characters of the lower Urodela, and show no evidence of the largely-developed branchial apparatus which is so characteristic of the latter. Thus, if upon such slender evidence as exists, it is justifiable to speculate at all concerning the &quot;phylogeny &quot; of the Amphibia, the most probable conclusion appears to be that the Labyrinthodonta, the Urodela, and the Anura diverged from one another at a very early period of geological history; while, possibly, the Peromela are the last remnants of the peromelous modification of the Labyrintho dont type. With respect to the origin of the amphibian stock itself, the fol lowing considerations appear to be of fundamental importance : 1. The early stages of development of the Amphibia do not resemble those of any known Ganoid, Telostcan, or Elasmobranch fish, and are similar to the corresponding stages of the Marsipobranchii. 2. The skull of the lowest Urodela has, in some respects, advanced but little beyond the Marsipobranch stage. In the higher Urodela there are numerous points of resemblance with the Ganoids. The skull of the tadpole, on the other hand, has much in common with that of Chimcera (as Miiller has pointed out), and with that of the Dipnoi, while the chondrocranium of the adult frog has many singular affinities with that of the Elasmobranchii, and particularly of the Rays. 3. The only Vertebrata, besides the Amphibia, which have transitory external gills are the Elasmobranchii, the Dipnoi, and perhaps some Ganoids. 4. The only fishes iu which the cere bellum is rudimentary are the Marsipobranchii and Ganoidci. 5. The only fishes in which the amphibian and embryonic connection between the male reproductive organs and the renal efferent ducts is observed are the Ganoids. 6. The only fishes which have a &quot;pylangiurn,&quot; with valves disposed as in the Amphibia, are the Ganoids, Elasmobranchs, and Dipnoi. 7. The only fishes which pos sess morphological (Polypterus) or functional (Dipnoi) lungs are the Ganoids and Dipnoi. The conclusions suggested by these facts appear to be that the Amphibia took their origin from some primordial form common to them, the Elasmobranchii, the Ganoidci, and the Dipnoi; and that the main distinction by which their earliest forms were marked off from those of the other groups, was the development of that peutadactyle type of limb, which is common to all the higher Vertebrata. And seeing that the Elasmobrauch, Ganoid, and Dipnous types were fully differentiated from one another in the Devonian epoch, it is reasonable to believe that the existence of the Amphibia, as a group, dates back at least as far as that remote period of the earth s history. TAXONOMIO SYNOPSIS or THE AMPHIBIA. I. THE URODELA. A. Branchiae persistent throughout life. (PcrennibrancJiiata.) 1. Trachystomata. Skull elongated ; pracmaxillce and den- tary piece of the mandible provided with horny plates ; premaxillae not ankylosed ; no nasal bones, but ossi fications between the ascending processes of the pre- maxillee ; maxillte rudimentary or absent ; palatines small, oval, and beset with &quot;dents en brosse;&quot; ptcry- goid absent ; four persistent branchial arches ; pelvic arch and limbs absent. Siren. 2. Proteida. Skull elongated; premaxillse and dentariea dentigerous ; maxillae rudimentary or absent ; pre maxillae not ankylosed ; no nasal bones ; palatines bearing a single row of teeth, and coalescent with the pterygoids ; three persistent branchial arches ; both the pectoral and the pelvic arches and limbs developed. Proteus, MenobrancJ^us. B. Branchiae caducous ; gill-clefts persistent. (Derotrcmata. ) 3. Amphiumida. Skull elongated; premaxillaj and den taries dentigerous ; maxillse large ; premaxillse anky losed ; large nasal bones ; palatines absent ; pterygoid present, elongated ; a basinyal cartilage ; four persis tent branchial arches ; both the pectoral and the pelvic limbs developed, though very small. Amphiuma. 3 &quot; Description of the Vertebrate Kemains from the Jarrow Colliery,&quot; by Prof. Huxley, F.R.S., Transaction* of the Royal Irish Academy, vol. xxiv. 1867, i l. xix. fig. 2.