96 PALAEONTOLOGY OF ILLINOIS. 



in the line extending from the longest and most acute angle to 

 the opposite margin ; crown-surface sometimes simply arched, 

 more generally marked by 1-3 prominent ridges, running from 

 the basal margin toward the longest angle. In some species 

 the triturating surface is also undulated by a series of trans- 

 verse obtuse ridges, parallel with the basal margin, and mostly 

 confined -to the basal portion of the tooth. The crown-surface 

 is uniformly punctate, the size and form of the pores varying 

 on the different species. 



This genus is created to receive a group of teeth, largely represented in the 

 collection before us, and of which some species have been described heretofore 

 in Europe, under the names of Cochliodus and Poecilodus. It is evident, how- 

 ever, that they should be separated from Cochliodus, as they are more triangular 

 in form and generally much less curved; and it will be seen, by a comparison 

 with Poecilodus, that while the forms of some species approach that of Poecilo- 

 dus, they are distinguished from that genus by the character of the surface. 

 In all the true Pcecilodi the enameled surface is corrugated by distinct trans- j y 

 verse ridges, which occupy all the crown face from base to summit of the teeth, / 

 and give them a peculiarly ornamented appearance. [See plate VIII, figs. 13 

 and 14.] In two foreign species of Poecilodus (P. augustus and P. marginatis) 

 the transverse rugae are unbroken undulations and so different from those in 

 the species cited as to render it doubtful whether they shonld be included in 

 the same generic group. One species of Deltodus now described and figured 

 (Z). undulatus, plate IX. fig. 5,) is also undulated, but not in a manner and to a 

 degree inconsistent with its association .with the smoother species with which 

 we have classed it. Another species, however (ZX cingulatus, figure 6), presents ' 

 some characters which doubtless will be considered to have generic value, and 

 we only refrain from separating it from Deltodus, at the present time, from an 

 unwillingness to make a single and imperfect tooth the basis of a generic 

 description. In that tooth the crown surface is composed of alternate trans- 

 verse bands of enamel and dentine, these bands being of nearly equal width 

 and running quite across the tooth, and extending from base to summit. 



It is almost eertain that the triangular teeth which we have referred to the 

 genus Deltodus were but a portion of the dentition of the fishes which wore 

 them, as they are undoubtedly the homalogues of the larger teeth of Cochliodus; 

 but we can not, as yet, determine the character, number or position of those 

 with which they were associated. 



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