36 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.37. 



Viewed from above, the tooth border is straight, but there is a twist 

 in the lower part of the dentary which throws the anterior ventral 

 border in toward the median line. The anterior end presents a sud- 

 den incurvature toward the symphysis, and, as in /Sphenodon, the 

 rami of the mandible aj^pear to have been united by ligament at their 

 anterior extremities only. 



The inner and superior angle of the symphysial end of each ramus 

 appears to have been separated from its fellow by a slight interspace 

 above the symphysis. On the superior surface of the dentary at this 

 l)art there is a well-developed subconical incisor, the base of which 

 ajDpears to be embedded in the substance of the dentary. The ventral 

 border of the dentarj^ is sinuous. Just posterior to the s^nnphysial 

 end it has a vertical depth of 5 mm. Posteriorly the bone gradually 

 widens, reaching its maximum dej)th below the second tooth from the 

 last of the dental series. There is a well-developed coronoid process, 

 as shown in fig. 1. The dorsal portion of this process is missing 

 from the specimen, but its outline is indicated from the impression 

 left in the matrix which (n'iginally inclosed the sjjecimen. 



Fig. 1. — Left dentary of opisthias karus. cat. no. 1!S60 u.s.n.m. x3 viewed from 



the internal side. 



Posterior to the coronoid process the dentary tapers to a thin 

 pointed end, which articulated with the posterior elements of the 

 mandible. The external surface of the dentary is gently rounded 

 from above dowuAvard. Below the dental border, on this side, runs 

 a low, curved, longitudinal ridge (fig. 1, pi. 11), probably for the 

 attachment of the lijo. 



On the internal side (fig. 1) is a pronounced mandibular groove 

 extending forward from below the coronoid process nearly to the 

 symphysis, where it fades out. 



The teeth are acrodont (i. e., ankylosed to the summit of the jaw), 

 as in Sjyhenodon. 



In the holotype there are, besides the incisor mentioned, 12 teeth, 

 preserved. A fracture of the bone appears to have destroyed one 

 tooth, so that in this individual there were 14 teeth present in the 

 mandible. In the paratype (Cat. Xo. 2858) the complete dental 

 series appears to be present, and I am able to distinguish 19 teeth 

 in all. Giinther has found an equal number in the dentary of 

 iSpheitodou. I 



