VERTEBRATES. 285 



inferior surface excavated immediately behind the marginal border, 

 with a beveled space extending along the posterior margin, superior 

 surface more or less convex and beveled to the posterior edge. Crown 

 well-defined from the base, constricted in front and laterally, forming a 

 subacute crest, gradually increasing in prominence from the extremities 

 towards the middle, culminating in a more or less prominent antero- 

 posteriorly compressed median cone ; both coronal surfaces are marked 

 with more or less regular, vertically or radiatingly disposed plica? or 

 ridges, the apical terminations of which produce a delicate, sometimes 

 strong, denticulationin the crest; crown enameled. 



In the group of teeth here designated, the crown bears a strong 

 resemblance to that of Hybodus (especially H. plicatilis, Agassiz, of the 

 European Muschellcallc), and also Mascalodus, while its affinities with 

 Orodus are, perhaps, less striking. Indeed this peculiar group appa- 

 rently presents a Hybodus-Mlie crown planted upon a Cladodus base ; or, 

 in other words, a combination of characters which, separately consid- 

 ered, find their peculiar development in groups or genera pertaining 

 to remotely separated geological epochs. That it constitutes a type 

 holding a position intermediate between Hybodus and Cladodus, com- 

 bining in its external features characters which are peculiar to one or 

 other of those genera, and thence forming, as it were, a connecting link 

 between these geologically widely separated groups, seems to be most 

 apparent. 



In his great work on the Fossil Eishes of the Old Red Sandstone,* 

 Professor Agassiz has described a form of teeth under the name Cla- 

 dodus simplex, from the Devonian in the environs of St. Petersburg, 

 which seems to exhibit precisely the same coronal features pertaining 

 to the teeth under consideration. But, since its base is imperfect, 

 important characters are wanting, without which its identity with the 

 present genus cannot be determined. I have also recently examined a 

 specimen obtained by Robert Drinkwatek, from the Coal Measure 

 shales near. Manchester, England, which appears to be generically iden- 

 tical with the teeth designated al>ove, though it differs in having the 

 lateral portions of the crown occupied by a few very strong, conical 

 tuberculattous, the median cone relatively low, and ornamented by a few 

 strong, sharp, bifurcated vertical costse. I have not been able to ascer- 

 tain whether the form here indicated has been described. 



* Pois. Foss. du Vieux Ores-rouge, p. 124, Tab. 33, figs. 29, 30, 31. 



