VERTEBRATES. 65 



Not infrequently individuals of the median forms are met with 

 which show one or more partially detached, laterally elongate teeth 

 at the outer extremity of the plate, which is always abruptly trun- 

 cated, leading to the inference that these median plates themselves 

 were, in the earlier stages of development, made up of series of 

 teeth. On the other hand, certain forms occur which have been 

 noticed under various generic designations (Desmiodus, Orodus), 

 which are composed of series of transverse coronal crests, firmly 

 soldered into a continuous plate at the impingement of their bases, 

 in which condition they bear a remarkable resemblance in outline 

 and direction of inrollment, to the above mentioned median forms. 

 But beyond deducing certain permissible conclusions bearing on the 

 probable affinities of Psephodus, the meagre array of data is deemed 

 insufficient to warrant, at the present time, the critical relegation of 

 the diverse dental materials alluded to in this connection. 



In American geological history, Psephodus was amongst the earli- 

 est representations of its order in the Lower Carboniferous period, 

 its first occurrence dating from the Kinderhook epoch. Although 

 the succeeding Burlington, Keokuk and Warsaw formations have as 

 yet afforded no evidence of the presence of the remains of the genus 

 (if we except the occasional appearance of Helodus planus-like forms), 

 its forms are sparingly met with in the St. Louis deposits, and in 

 the super-adjacent Chester division several forms, probably pertain- 

 ing to a single species, are conspicuously prevalent. The various 

 species occurring through so great an interval of time, ranging from 

 the earliest to the latest epoch of the period, exhibit an extraordinary 

 persistency of the type without parallel amongst the Cochliodonts, 

 if we except the doubtful relations of the later introduced species 

 provisionally identified with Vaticinodus. From this point of view 

 the genus would appear to be prominently isolated so far as relates 

 to precursor or subsequent derivatives. Of the former we have not 

 even the vaguest intimation, and the latter, even under the most liberal 

 interpretations, have undergone excessive differentiation. The pres- 

 ence, in the extreme anterior portion of the jaw of Cochliodus, of 

 transverse coronal ridges, if not of isolated teeth, in series corres- 

 ponding to those abaft and forward of the convolute median plate 

 in Psephodus, offer tangible evidence of their probable origin, though 

 their relations are less intimate than might be expected in groups 

 almost coeval in their inception. 



