INTRODUCTORY. 59 



In the majority of cases the remains of fishes are found in definite, 

 readily identifiable horizons, indeed chiefly in a limited stratum, 

 where they are crowded together forming veritable bone-beds, though 

 by no means thus restricted, as their remains are found more or 

 less scattered in many formations. Yet the latter occurrences are, 

 as might be expected, comparatively rare. 



The earliest genus, Psephodus, whose relations with the more 

 typical representatives of the family may be a matter of reasonable 

 doubt, had its beginning in the oldest or Kinderhook formation, 

 only ceasing in the latest or Chester epoch of the Lower Carbonifer- 

 ous period. Near the close of the Kinderhook epoch Vaticinodus 

 was introduced, authentic representatives of which continued to exist 

 during the Upper Burlington epoch. Tteniodus, whose relations 

 seem to be nearest Psepkodus of all the true Cochliodonts, is first 

 met with in the Keokuk formation, continuing thence into the 

 Chester, where it is last seen. During the Upper Burlington epoch 

 DeUoptychiw was introduced, which appears to be a modification of 

 Vaticinodus, as is also Stenopterodus, which originated at the same 

 time, and whose remains occur in each succeeding formation as late 

 as the St. Louis, while Deltoptychius continued to exist during the 

 deposition of the Chester limestone. In point of time, also, Sanda- 

 lodus began contemporaneously with the latter genera, with which its 

 affinities appear to be most intimate, although it presents a more 

 marked modification of generic characteristics than those distin- 

 guishing the three last named genera, one from the other. In 

 OrthopleurodiLs these modifications are carried to an extreme, as 

 especially notable in the maxillary terminal form, though its man- 

 dibular terminal teeth are very similar to the homologous form of 

 Deltoptychius, while the median form of the same jaw is intimately 

 allied to that of Sandalodus. Pertaining to the same category are 

 the forms of Xystrodus, and, perhaps, Tomodus, which apparently 

 was the contemporary of the above mentioned genera, its remains 

 being first encountered in the Upper Burlington, and only ceasing 

 in the lower Coal Measures. It presents the simplest form of the 

 group to which it especially pertains, and throughout its extensive 

 vertical range or distribution in time, it maintains its distinctive 

 peculiarities with remarkable persistency. 



