Lacertilia. The zygomatic arch of the Mammalia is formed (cf. also Agamidae) out of the supratemporal arch of Sphenodon, after the loss of the postorbital element and of the quadrato-jugal, the squamosal gaining connexion with the upper, not posterior and ventral, branch of the jugal or malar bone.
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Fig. 17.—Ventral Aspect of Skull of Chelys matamata. bo, basioccipital; bs, basisphenoid; mdl, mandible; oh, opisthotic; pl, palatine; pm, premaxilla; po, proötic; pb, pterygoid; q, quadrate; s, squamosal; v, vomer. |
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Fig. 18.—Lateral Aspect of Skull of Chelys matamata. an, angular; ar, articular; bo, basioccipital; d, dentary; op, opisthotic; m, maxilla; pa, parietal; pm, premaxilla; pr, prefrontal; ps, postfrontal; pt, pterygoid; q, quadrate; s, squamosal; sg, supra-angular. |
The mandibular halves form a complete osseous symphysis, the only instance in reptiles; all the other elements retain their sutures. The articular portion of the articular bone forms several shallow cups and a slight anterior knob, best developed in Chelone. The angular bone does not help to form the posterior upper angle. The coronoid, or complementary element, is often small; the supra-angular and the splenial or opercular are always present, mostly also a pre-splenial wanting in Testudinidae (cf. G. Baur).
The hyoid apparatus is well developed, and sometimes assumes large dimensions, especially in Chelys. The two pairs of “horns” are the first and second branchial arches, whilst the hyoid arches are reduced to a pair of small, frequently only cartilaginous nodules, attached near the anterior corners of the basis linguae, which generally fuses with the os entoglossum in the tip of the tongue. In Chelydidae the long median basal or copular piece forms a semi-canal for the reception of the trachea.
In the skull of the Lacertilia the arcades over the temporal region vary much in composition and numbers. There are at most two arcades and two windows. First the posttemporal arcade, enclosing the posttemporal fenestra, which is framed mainly by the large paroccipital process below and the long parietal process above, both meeting distally, and the quadrate is carried by the paroccipital process. In the corner, in front, where the three bones meet, lies the squamosal, connecting parietal and quadrate. This squamosal, when not too much reduced, has an upper parietal and an anterior horizontal arm; the latter is essential for the formation of the second horizontal arcade, which makes the lower border of the supra-temporal window. The infra-temporal arcade, namely a quadrato-jugal+jugal arch, is absent in all Lacertilians owing to the complete absence of the quadrato-jugal element.
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Fig. 19.—Skull of Chlamydosaurus kingii (old male), showing much differentiated teeth. 1, ventral aspect; 2, posterior; 3, profile, showing the enormous process at the hinder end of the lower jaw. |
In Heloderma and Geckos the post temporal is the only arcade. In the Amphisbaenids and in Aniella, practically also in Anelytropsis, all the arcades are lost. All the other families of lizards and the chameleons have two arcades. We begin the description of the horizontal arcade with those families in which it is most complete, and most like that of Sphenodon. In Varanus it is formed by four bones. The postfrontal is short; to it is attached the post orbital, which sends a long horizontal process to join the squamosal[1] splint, and this connects with the
- ↑ There is a much-debated question of the homologies of the one or two elements, both apparently membrane bones, which connect the upper end of the quadrate with the parietal and with the supratemporal arch. The question becomes acute in the snakes, whether the single element connecting skull and quadrate has to be called squamosal or supratemporal. Space forbids here to expound the matter, which has been very ably reviewed by S. W. Williston (“Temporal Arches in the Reptilia,” Biolog. Bulletin, vii. No. 4, 1904, pp. 175-192; cf. also F. W. Thyng, Tufts College Studies, II. 2, 1906). About ten different names have been applied to these two elements, and two, namely, squamosal and supratemporal, are being used quite promiscuously. When only one element is present, the present writer uses the term squamosal, and there are reasons making it probable that this element is the squamosum of mammals. When both elements are present, the more ventral or lateral of the two is termed squamosal, that which always helps to form the
