VERTEBRATES. 95 



oblique antero-lateral border rapidly converging toward the outer 

 extremity, which presents the same degree of inrollment observed 

 in the preceding posterior form. Coronal region presenting a series 

 of transverse ridges, which individually possess coronal characters 

 ascribable to certain forms of Helodus, a medium-size specimen 

 showing five such coronal ridges, whose longer axes correspond to 

 longer diameter of the posterior tooth. The individual ridges rise in 

 an even convex surface behind, culminating in an obtuse even crest, 

 the outer face moderately concave vertically, and invested in the 

 enamel layer, save along the crests, which reveal the relatively 

 coarse punctate structure. Breadth of series across inner margin 10 

 m m. ; length along postero-lateral border 11 m in., corresponding to 

 that of the anterior border of the posterior tooth. The proportionate 

 dimensions of the individual coronal ridges are represented in the 

 illustrations. 



Of the forms above noticed and associated under the same specific 

 designation, the collections contain a fair suite of representatives, 

 amongst which the teeth referred to the posterior position upon the 

 lower jaw are in about double the numbers of those belonging to 

 the maxillaries ; while of the small anterior, or median dental series 

 of the upper jaw, only a single specimen is known that discovered 

 by Mr. Wachsmuth, at Danville, Iowa. With regard to the latter, 

 the resemblance it bears to Helodus elytra, N. and W., of the same 

 geological horizon, creates a suspicion of the specific identity of 

 similar Helodns-like teeth with the form here alluded to. The 

 original specimen of Helodus elytra, however, apparently shows a 

 series of independent, contiguous teeth, in their relative natural 

 position, but with their coronal crests worn down almost even with 

 the basal margins, so that it is impossible to determine the coronal 

 contour, although, as has been stated, we strongly suspect the form 

 is identical with teeth, perfect specimens of which have been de- 

 scribed under other designations. The consolidation along the basal 

 impingement of the separate teeth is precisely what has been noticed 

 in connection with other allied forms of this family, 'and although a 

 matter of biological interest and importance, it should not militate 

 against the recognition of the specific identity of series of isolated 

 teeth and those that are joined by their bases into a solid dental 

 plate, as is the case in the above form. 



The maxillary posterior teeth are seldom preserved entire, and the 

 attenuation of the anterior region exhibits unmistakable evidences 

 of the excessive attrition these teeth were subjected to during the life 



