VEETEBEATES. 137 



General outline trigonal, lateral diameter proportionately short, 

 compared to the length, which it but little exceeds ; general course of 

 inner margin nearly at right-angles with the antero-lateral border, 

 somewhat sinuous and broadly rounded, and somewhat sharply so 

 in passing the angle of the principal coronal prominence, thence 

 pursuing a slightly-curved course into the produced posterior angle ; 

 antero-lateral border nearly straight or very gently-curved sigmoidally 

 in passing into the extremely-inrolled outer extremity ; basal portion 

 nearly vertical, very deep, and denned above by the usual narrow 

 enamel fold, which forms an inbeveled coping projecting considerably 

 beyond the basal wall; postero-lateral border rapidly converging to- 

 ward the outer margin, making an angle of about 50 with the anterior 

 border; condition of coronal fold and basal edge not determinable. 

 Coronal region strongly arched in the direction of inrollment, and 

 principally occupied by the longitudinal prominences ; principal ridge 

 nearly median, strongly arched transversely and defined by a more 

 or less distinct angulation from the broad, posterior depression, 

 which is bordered by the gentle acclivity rising into the postero- 

 lateral border of the alate expansion ; anterior ridge almost equally 

 prominent and similar in contour to the principal ridge, from which 

 it is separated by a deep depression of nearly equal breadth, with 

 very steep, concave walls, slightly flattened along the rounded crest, 

 the slope in front descending, in a similar slight concavity, to the 

 antero-lateral border, which is margined by a very narrow fold of the 

 coronal enamel. Surface minutely and densely punctate, with faint 

 transverse lines along the less worn inner margin, otherwise des- 

 titute of the rugosities common to nearly all its congeners. Trans- 

 verse diameter at the inner margin about 35 mm. ; length of antero- 

 lateral border to point of inrollment, 25 mm. ; ditto along principal 

 coronal ridge, about 30 mm. ; depth of antero-lateral border, 5 mm. 



The outer portion, as also the postero-lateral border, is mutilated, 

 exposing to view the remarkable inrollment which amounts to one 

 and a half revolutions, with a short convoluted apex. The form 

 described differs in so marked a degree from other species of the 

 genus as not to require detailed comparison ; it is chiefly distinguished 

 from allied species by the regularly arched coronal ridges, deep 

 narrow median depression, and abbreviated transverse diameter. 



This largest American representative of the genus, and I believe 

 the largest species known, is dedicated to Professor Worthen, the 

 venerable director of the geological survey. 



Geological position and locality: Chester limestone, Chester, 111. 



