VERTEBRATES. 173 



extremity at an angle of about 25. A large example of the trigo- 

 nal form measures in length along the straight border 31 mm. ; 

 breadth across inner margin 27 mm., more or less; lateral borders 

 converge at angles of 30 to 40. 



The above notices of the forms associated by Professor Agassiz 

 under the specific designation Tomodus convexus, will, perhaps, also 

 serve to convey a tangible idea of the distinctively generic charac- 

 teristics ascribable to the genus Tomodus. Its relations are most 

 intimate with Xystrodus, from which it chiefly differs in the con- 

 spicuous convexity of the coronal ridge, the extremely abrupt artic- 

 ular border, and the absence of transverse disposition of the punctae 

 of the triturating surface. Our knowledge of the genus is so imper- 

 fect that we may have overlooked important generic characters, 

 which the specimens themselves might reveal at a glance. Viewed 

 in accordance with the facts elicited from the data within reach, 

 we are led to suspect some confusion exists in the interpretations 

 of the limits of the genus, and that in consequence forms have been 

 referred to it which have no intimate congeneric relationship with 

 Tomodus convexus. Of the latter, the species described by Dr. 

 L. de Koninck in his great work on the Belgian Carboniferous 

 limestone fauna,* under the names Tomodus Craigi, and T. lacini- 

 atus, certainly show a wide departure from the typical species 

 T. convexus, Ag., while their relations with Psephodus are strongly 

 suggested to judge from the admirable figures illustrating the Bel- 

 gian teeth. The specific identity of the two forms ascribed to 

 T. convexus may be also reasonably questioned, since both have the 

 general conformation of mandibular teeth, and it is difficult to con- 

 ceive how they could have been associated on the same jaw. 



The American form mentioned in this connection, and which is 

 provisionally referred to Tomodus, is the sole representative of the 

 genus thus far brought to light in the Carboniferous formations of 

 this country. The genus is Carboniferous, restricted so far as is 

 known to the earlier group of the series. 



TOMODUS ? LIMITABIS, St. J. and W. 



PI. XIII. Fig. 12. 



Teeth small, longitudinally elongate, cuneiform in outline, rather 

 strongly arched in the direction of inrollment. Antero-lateral bor- 

 der gently curved sigmoidally from the obtuse inner angle to the 



*Faune du calcaire carbonifere de la Belgigue, Premiere partie, 1878. p. 00. 



