182 



alveolar border only; the maxillaries meet above it. The prefrontals have coalesced with 

 both lachrymals and nasals : the latter terminate in points overhanging the external nostril. 

 The alveolar borders of both upper and lower jaws show a regular series of vascular pits or 

 foramina, indicative of the primitive separate matrices, like those of teeth, which laid the 

 foundation in the young animal of the continuous horny coverings of the jaws. 



The specimen from which this and the following parts of the skeleton have been 

 prepared was presented by Capt. Sir Everard Home, Bart., E.N., F.E.S. 



923. The atlas, or first vertebra of the neck, of the same Trionyx. 



The hypapophysis presents four articular surfaces : one, anterior, concave, for the lower 

 part of the occipital tubercle ; one, posterior, flat and subcircular, for the proper centrum 

 (odontoid) ; and two, lateral and superior, for the neurapophyses : these are joined together 

 above the neural arch by suture: the anterior articular surfaces are cut obliquely from 

 their fore part, and are adapted to the sides of the occipital tubercle formed by the ex- 

 occipital elements. Two corresponding oblique surfaces behind these articulate with the cen- 

 trum : the posterior zygapophyses are very long, and are directed backwards, with the arti- 

 cular surfaces looking downwards and inwards. The centrum, or 'odontoid,' presents a 

 subcubical form, with a small subcircular surface on its lower and fore part for articulating 

 with the hypapophysis ; above this, a transversely expanded portion, with a convex surface 

 adapted to the bases of the neurapophyses, and completing the cup for the occipital condyle ; 

 and at the back part an excavation to articulate with the anterior convexity of the succeeding 

 centrum. 



924. The second cervical vertebra of the same Trionyx. 



It presents a convex anterior surface for articulation with the true body of the atlas, which, 

 in higher animals, is united thereto as the ' odontoid ' process. 



925. The third cervical vertebra of the same Trionyx. 



It is much elongated ; the suture between the centrum and neural arch remains ; the cen- 

 trum is convex anteriorly, concave behind, the lower part of which concavity is formed by an 

 epiphysis, analogous to the 'wedge-bone,' or hypapophysis, of the Ichthyosaurus. 



926. The fourth cervical vertebra of the same Trionyx. 



It resembles the preceding ; but the ridges extending upon the posterior zygapophyses are 

 stronger, and the transverse processes more developed. 



927. The fifth cervical vertebra of the same Trionyx. 



The articular surfaces of the zygapophyses here begin to be concave in one direction, con- 

 vex in the other, so as to produce an interlocking joint. 



