VEETEBRATES. 355 



eric distinctness from the first noticed form, H. dentatus. Thus limited 

 and denned as above, Rarpacodus constitutes a well characterized gene- 

 ric group, the affinities of which would appear to be near Chomatodus, 

 as indicated not only by the low, compressed crown, but also the con- 

 formation of the basal region. 



Haepacodtts occidentalis, St. J. and "W. 



PI. Xa, Fig. 2. 



Teeth small, laterally elongated, gently arched outward along the con- 

 cave border and also along the crest. Crown compressed along the 

 crest, expanded below in the concave face, deeply pectinated with from 

 ten to twelve rounded denticulations, the intervening sulci extending 

 downwards nearly to the basal border, very gradually diminishing in 

 size towards the extremities, which are abruptly rounded ; the convex 

 face forms a long, narrow crescent-shaped area, more or less strongly 

 curved downward at the extremities, rising vertically from the basal 

 margin ; concave face about a third higher than the opposite side, 

 gently depressed, and laterally and vertically gently arched, basal bor- 

 ders prominent, abruptly curved upward at the extremities, and contin- 

 uous with the less prominent coronal border of the convex face, in both 

 faces abruptly inflexed to the basal region. Cororfal surfaces smooth, 

 the crest sometimes delicately striated. Base moderately thick, late- 

 rally nearly as long as the crown, which it equals in depth, the deep side 

 beneath the convex crown face gently excavated and nearly in the same 

 vertical plane, a third deeper than the opposite side, which is somewhat 

 more deeply channeled, inferior surface plane, obliquely beveled to the 

 acutely angular posterior (?) border, lateral angles well-defined. Greatest 

 diameter of a medium sized specimen .25 inch, vertical elevation of 

 tooth .10, anteroposterior diameter at base of crown .05, elevation of 

 concave crown-face .07 inch. 



The above form is based upon specimens discovered by Mr. Van 

 Hoene, in whose collection there is a nearly perfect example, while we 

 have obtained other more or less perfect examples, which exhibit but 

 slight individual variation, and so well-marked is the form that even 

 fragmentary specimens are readily determinable. 



Position and locality : Upper beds of the St. Louis limestone ; Alton, 

 Illinois, and St. Louis, Missouri. 



Habpacodtjs comp actus, St. J. and W. 



PL Xa, Pig. 1. 



A specimen obtained by Mr. Woethen from the Chester formation, 

 which exhibits the concave aspect of the tooth, constitutes a second and 



