CHONETES 85 



gular or sometimes approaching rectangular, the auricular portion small 

 and not sharply differentiated from the body of the shell, the lateral mar- 

 gins nearly straight or slightly concave posteriorly, convex anteriorly 

 and passing without break into the regularly rounded anterior margin, 

 the length usually a little less than two-thirds the width. The dimensions 

 of three specimens are : length 6.3 mm., 6.8 mm., 10.5 mm. ; width 8.9 mm., 

 9.5 mm., 13.5 mm. ; convexity 2.6 mm., 3.5 mm., 5.8 mm. 



Pedicle valve most convex near or a little posterior to the middle, the 

 surface curving abruptly to the beak and a little more gently to the ante- 

 rior margin, compressed towards the cardinal extremities but with the 

 auriculations small, not sharply differentiated, and convex from the car- 

 dinal margin posteriorly ; mesial sinus obsolete ; the beak small, produced 

 slightly beyond the cardinal margin, the umbonal surface smooth for a 

 short distance from the beak; the cardinal area narrow, slightly concave, 

 extending posteriorly in nearly the plane of the valve, the delthyrium 

 small, the cardinal margin sharply defined, bearing two or three spines on 

 each side of the beak, all of which are directed obliquely outward. In- 

 ternally the muscular scars are rather strongly denned, and are divided 

 along the median line of the valve by a median septum which varies in 

 strength in different individuals from a slightly raised line to a strongly 

 raised, narrow ridge ; beyond the limits of the muscular scars the surface 

 is papillose throughout. 



Brachial valve deeply concave, the greatest concavity near the middle, 

 becoming flatter and only gently concave in the auricular regions towards 

 the cardinal extremities ; the cardinal area narrower than that of the op- 

 posite valve and meeting it in an obtuse angle ; the costae do not extend to 

 the beak but leave a small, smooth area at the apex of the valve in the 

 middle of which, adjacent to the cardinal margin, a small, node-like prom- 

 inence is sometimes present. 



Surface of both valves marked by rather broad, depressed, rounded 

 costae much broader than the intervening furrows, about three of which 

 occupy the space of one millimeter, on the pedicle valve they increase by 

 division and on the brachial valve by implantation ; crossing the costae 

 are exceedingly fine, raised concentric lines, 10 or more in the space of 

 one millimeter, which are obsolete in the furrows between the costae, and 

 give to the costae a distinctly crenulate appearance ; the costae do not 

 originate at the beak, but at a little distance to the front, and they become 

 faint or sometimes obsolete towards the cardinal extremities. 



Remarks. This species is a common member of the fauna of the Kinder- 

 hook oolite bed at Burlington, Iowa, where it is associated with the less 

 common C. burUngtonensis, from which it differs in its greater convexity, 

 its greater proportional length, its broader and less numerous costae, and 

 the transverse crenulations of the costae. In the transverse markings of 



