LOGAN: CRETACEOUS INVERTEBRATES OF KANSAS. 93 



Length 40 mm 



Width 20 " 



Height '. 10 " 



Locality: White Rock creek, Jewell county, Kansas. 



Geological horizon: Rudistes beds, Niobrara Cretaceous. 



Remarks. — It may be found expedient to form a new genus for these forms. 

 They certainly present as great generic differences as Gryfhea and Exogyra, but 

 the necessity for such genera has been questioned. Furthermore it is the belief of 

 the writer that as the science of Palaeontology advances there will develop a tend- 

 ency toward the contraction rather than the expansion of the number of genera 

 and species. Should such a condition obtain this genus, if created, would in all 

 probability be among the first to fall. 



Ostrea attenuata, n. sp. Plate XXII, Figs. 2, 4. 



As will be readily seen from the figures this species is very close- 

 ly allied to O. incurva, figures of which are found on the same plate. 

 The forms are so nearly alike that further description is unneces- 

 sary as an enumeration of the points of difference will suffice to give 

 an understanding of the form. 



The shell descends from the central line of convexity by equal 

 slopes; the beak is not turned laterally; the dorsal and ventral 

 borders are symmetrical. Otherwise the shells are the same. 



It may be that the differences arise from deformations of O. in- 

 curva, but it is not to be expected that there would be -such a uni- 

 formity of deformation as is exhibited in the numerous forms col- 

 lected. They may, however, be merely varieties of the one species. 

 They were collected from the same geological horizon and from the 

 same locality. 



Ostrea crenula, n. sp. Plate XXI, Figs. 7, 8, 9. 



Shell (right valve), small to medium in size, thin, very convex, 

 ovate in marginal outline; surface rising abruptly from beak, then 

 sloping backwark and upward more gradually to one-third the 

 length of the shell and from that point sloping very gradiiall)/ 

 downward to the posterior border. From the central line of con- 

 vexity the sides of the shell fall away rather abruptly, the dorsal 

 more abruptly than the ventral. The exterior surface is marked 

 by ridges and grooves which at the margin assume the form of 

 crenulations. Ventral border nearly straight; posterior border 

 short, rounded, crenulate; dorsal border forming a convex curve; 

 muscular impression sub-central. 



The form of the lower valve cannot be made out as it is crushed 

 inside the upper one, but its surface shows striations and its margin 

 is somewhat crenulate. The shell was evidently attached by the 



