Page:Synopsis of the Exinct Batrachia and Reptilia of North America. Part 1..pdf/69

 AND AYES OF NORTH AMERICA. 6:i This genus presents the same peculiarity of dentition as the Plerodon Meyer (Diplocy-nodus Pomel) of the Etuopean Miocenes. The P. pleniclens, and P. ratelii are both of the Crocodilian type of cranium, the rami of the mandible with curved extremity and short symphysis, while Thecachampsa is a gavial, with very long symphysis and slender muzzle. I have seen but one cervical vertebra from American tertiaries, and that is of the type of Thoracosaurus ; hence this character cannot be certainly ascribed to Thecachampsa. Three species appear to exist in our Miocene beds. The T. sicaria indicates in its slender inaudible one character of the genus ; it shows the surface to have been ridged and pitted as in other Croeodilia. The 'I'. antiqua Leidy indicates in its dorsal vertebra, a smaller Wypapophysis than in the known species of Crocodilus. T. seiicodon Cope is only known from its teeth. The teeth of the three species may be thus distinguished. It must be mentioned that I have but one tooth of T. sicaria, three of T. antiqua and six of T. smicodon. In the first the tooth has a lenticular section a short distance below the tip, owing to the great development of the lateral cutting ridges, and the compression of the crown at their bases. In the other two, these ridges are much less deg eloped ; in 1'. antiquus they exist only towards the tip on the inner or concave face of the tooth, while in T. sericodon they extend more than half the length of the crown towaids the base, on the inner side. THECACJIAMPSA SICAItIA, Cope. Proceed. Ac. Nat. Sei., Phila., 1869, 8. This species is represented by a lumbar vertebra, an imperfect crown of a tooth, and a portion of the under jar. They were submitted to me by Philip T. Tyson, State Geologist of Maryland, who procured them from near the mouth of the Patuxent liver, along with the remains of Escluichtius, Physeter, and other Cetacea. The portion of mandible indicates an animal of a size considerably exceeding both the Gavial of India and the Thoracasaurus of the Cretaceous of this country. It contains all or parts of alveolae of six teeth. Opposite the fourth alveolus from the front, the margin diverges slightly from the median line, indicating the position of the distal extremity of the splenial bone. The slight degree of this obliquity indicates an extensive contact of these elements, and not a symphysis formed merely by union of the dentary elements as in Mecistops and Crocodilus. As no curvature appears at the anterior extremity of the fragment, and the alveolae are similar to those succeeding, it has evidently not been broken from the anterior portion of the symphysis. The nutritious canal of the ramus is thus nowhere exposed, but is enclosed in the long symphysis. The upper face of the ramns is convex, most so anteriorly. Its lateral and inferior face is more convex than hi other Gavials which I have noticed, especially posteriorly. Its surface is coarsely sulcate, and with numerous small foramina. A larger space than elsewhere is seen between the two median alveolae, which is occupied by a deep con-cavity for the reception of a large tooth of the maxillary series. This indicates an irregularity in the size of the teeth of that series, as in the Crocodiles, and not an equality as in other Gavials. On placing the fragment in position the teeth are seen to have diverged at an angle of 45°. The specimen had laid sufficiently long in the Miocene ocean bottom to have been fixed upon by barnacles and oysters, as a place of abode. That it had not remained unburied very long is evident from the small size which these parasites had attained ; and that it was buried in Miocene deposits and not worn by a more modern sea. is testified to by the Miocene shells (Turritella, etc.), whose fragments were removed from its cavities with the sandy clay of its place of burial. The teeth have been broken off in this rough contact with the elements, but I procured a large and characteristic portion of the crown of a successional tooth whose apex had attained to the level of the edge of the