VERTEBRATES. Ill 



however, that it has any closer affinity with the typical species of Psammodus 

 than with Cochliodus, Helodus, and perhaps other genera of the cestraciont 

 family. Doubtless, in time, material will be obtained which will permit future 

 palaeontologists to determine its affinities with more accuracy than can be done 

 at present. In the hope of hastening that time the figures and descriptions of 

 it are now given. We have some reason to believe, however, that we have, in 

 this and other teeth, from the same vicinity and geological level, representatives 

 of one or more genera, distinct from any hitherto designated by special names, 

 teeth which hold an intermediate place between Psammodm and Cochliodus. 

 These are in addition to that under consideration, and such as should probably 

 be referred to the same species, those we have designated by the names of 

 Psammodus semi-reticidatus, Trigonodas minor, T. major, several species of 

 Sandalodus, etc. Of these, some, at least, depart very widely from the tile-like 

 teeth of the typical species of Psammodus, and no one would think of including 

 them in the same generic group. Others, however, such as that under consid- 

 eration, show perhaps sufficient relationship in their microscopic structure, and 

 in their broad, more or less tubular forms, to the true Psammodi, to be appro- 

 priately classed with them until such time as the dentition of the fishes to , 

 which they belong shall be more fully made out. 



Figure 6, crown surface, from above; 6a, side view, natural size. 



Formation and locality : Keokuk limestone, Warsaw, Illinois. 



Genus TRIGONODUS, N. and W. 



Gen. Char. — Teeth thick and strong, sub-triangular in out- 

 line, strongly arehed in both directions, with a central boss or 

 prominence, from which one or more ridges descend to the 

 angles or to the larger side; triturating surface uniformly and 

 finely punctate ; inferior surface deeply concave. 



This genus is established to receive a number of teeth constituting two spe- 

 cies included in the collection, which, with the mieroscopic structure of Coch- 

 liodus, seem by their peculiarities of form to be essentially distinct from that 

 genus. They formed strong, crushing implements, doubtless serving the same 

 purposes with those of the large group of Cestraciont genera, with which they 

 are associated in the Carboniferous limestone. In their forms they show a 

 marked departure from the more cemmon types of Cestraciont teeth, and ap- 

 proach more nearly to that of some portions of the triturating apparatus of the 

 Chimaeroid fishes. But we should be unwarranted in inferring, upon such 



