Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/470

Rh 452 KEPTILES [ANATOMY. no fontanelle in the cranial roof. The basisphenoid is prolonged forwards into a long bony rostrum, on the upper surface of which the cartilaginous rods (the per- sistent trabecu- lae cranii) ex- tend forwards to blend with the median cartilage of the ethmoi- dal region. The supraoccipital is excluded from the the ol margin ui Fl(J 15 __g kuU of yi pera nasicomn (from nature), ar, foramen articular; ca, columella auris; d, dentary; /, frontal; larrnnm V.IT tVio '" maxilla; *>/ Pison fang; pm, premaxilla; pr, pre- L uy l / frontal; ps, post frontal; pt, pterygoid; q, quadrate; s, eXOCcipital. The squamosal; <, transversum. frontal bones descend laterally to the dorsum of this basi- sphenoidal rostrum and then turn inwards to meet together in the median lines on the floor of the cranial cavity. The parietals also descend laterally but unite with the basi- sphenoid by suture. There are a pair of nasals, and also lachrymals and postfrontals, but there is no jugal or quad- rate jugal. The palatines do not bound the posterior nares behind. They are widely separated, and their long axes are longitudinal. The maxillae may be long and may support a number of teeth, as in most non -venomous Snakes, or maybe very short and support a single large fang, as, e.g., in Vipers. The small-mouthed Serpents, or Angiostomata, have the pterygoids separated from the quadrates. The squamosal is small or absent, the quadrate being attached directly to the cranium. They have no postfrontal and no transverse bone, while the palatine bones have their long axis trans- verse, and meet, or nearly meet, on the under surface of the skull. The two rami of the mandible are also closely united together. In other details of cranial structure they agree generally with the Eurystomata. The hyoid is rudi- mentary, and only consists of a pair of cartilaginous threads, medianly united and lying side by side be- neath the trachea. In the Chelonians the skull presents cer- tain resemblances to that of the Crocodil- ians. Thus the qua- drate is immovably 6 ^ fixed and its upper part is joined by the pterygoid, and there is a quadrato-jugal bone. The pterygoids and palatines unite by suture and form horizontal plates be- Fl - 16. Dorsal aspect of skull of Testudo tabu- i ,1,. .. la ta (from nature), an, anterior nares;/, frontal, neata the basis cranii. on either side of which are the orbits, bounded fhpcp behind by ps, the postfrontal; bo, ba.sioccipital; ' ep, epiotic; so, supraoccipital; g, quadrate; , bones differ Widely squamosal; pa, parietal; po, periotic bones. from their homologues in the Crocodiles inasmuch as the posterior nares open in front of the pterygoids and are bounded by the palatines and the azygous vomer. The epiotic bone is generally quite distinct from the exoccipital. There are large parietals, which send down a process that serves the purpose of the absent alisphenoid. In front of this, the sides and front of the cranial cavity are unossified. There are a pair of frontals, and in front of them a pair of bones which bound the anterior nares above. There may

jpnv an d FIG. 17. Side view of skull of Testudo tabulata (from nature), an, angular; m; articular; d, dentary;/, frontal; j, jugal; m, mandible; n, naso-pref rental; pa, parietal; pi, palatine; ps, postfrontal; q, quadrate; gj, quadrato-jugal. be one or two premaxillae. Sometimes, as e.g., in Chelone, the parietal, post- frontal, j ugal, and squamosal send out plate-like" processes which unite and form a sort of false outer skull covering in the temporal fossa, and quite ex- ternal to the real outer wall of the cranial cavity. Thus the cranium of these Reptiles is, as it were, at the opposite extreme to that of the Serpents as regards massiveness, solid- ity, and the immov- ability of its several parts one on another. It maybe excessively flat and depressed, as in Chelydra. FIG. 18.- Ventral surface of skull of Testudo tabu- The dentary bone ^ a ^ a (f rom nature), bo, basloccipital; bs, basisphe- -, vi i noid; p, epiotic; m, maxilla; pi, palatine; pm, Ol the mandible IS premaxilla; pt, pterygoid; g, quadrate; gj, quad- azygOUS, as in Birds. rato-Jugal J *>, supraoccipital. The hyoid consists of two pairs of cornua, whereof the pm ps W Fio. 19. Dorsal aspect of skull of CheJys matamata (from nature), bo, basioccipital; eo, exoccipital; /, frontal; j, jugal; m, maxilla; pm, premaxilla; pa, parietal; pr, prefrontal; ps, postfrontal; pt, pterygoid; g, quadrate; *, squamosal; so, supraoccipital. anterior may or may not be the larger, attached to a
 * P