VERTEBRATES. 73 



an angle of 55 with the opposite border, with which it agrees in 

 other respects, although the basal portion is thicker and expanded 

 posteriorly beyond the limits of the coronal fold; inner margin at 

 right angles to the antero-lateral border, broadly arched from the 

 obtuse anterior angle round the base of the coronal prominence, 

 behind which it makes a slight concavity on the way to the sub- 

 acute posterior extremity. Coronal surface gradually rising into the 

 rounded crest which lies a little behind the median line, the poste- 

 rior slope slightly concave, the anterior slope even Jess so, alate 

 expansion obscurely defined. Surface marked by minute, wide- 

 spaced punctae. Greatest lateral diameter across the inner margin 

 13 mm. ; length along antero-lateral border 9. mm. 



The above described form is represented by a couple of nearly 

 entire teeth, in the collection of Mr. Van Home. They are closely 

 related to the corresponding form of the representative species oc- 

 curring in the Chester formation, which latter, however, is distin- 

 guished by the finer punctation, more oblique backward course of 

 the inner margin, and less central position of the coronal ridge. 



The St. Louis collections also afford many examples of the diverse 

 forms of teeth which have usually been identified with the genus 

 Helodus, but which, possessing certain characteristics in common 

 with unquestionable forms of Psephodus, seem to possess intimate 

 generic relations with the present form. The latter teeth may be 

 distinguished in a general way by their eccentric coronal prominence, 

 crenulated lateral borders and margin of the abrupt outer coronal 

 slope, presenting extreme variation in outline and coronal contour. 

 Not infrequent examples are met with showing the downward and 

 outward produced base characteristic of Lophodus, as obtains in 

 L. (Helodus) didymus, Ag. ; but more frequently the posterior basal 

 portion is inbeveled, indicating that the individual may have con- 

 stituted one of a series of teeth more or less perfectly joined at 

 their bases, forming a continuous plate the same as is known to 

 occur in individuals of Psephodus. Varying from the asymetrical 

 contorted forms to broad surfaced sublunate symmetrical teeth, 

 almost every conceivable variation attributed to this class of teeth 

 is observed amongst the St. Louis material, the same as occurs in 

 the Kinderhook and Chester collections. The specific identity of 

 these diverse forms with the teeth above described is in many in- 

 stances as clearly indicated as may be under the circumstances ; 

 but that all of them belong to a single species, we would not under- 

 take to affirm with equal confidence. 



