INVERTEBRATE PALAEONTOLOGY. 287 



spaces on the posterior and anterior cuds of the shell are only marked by 

 the strong concentric undulations. These costae are smaller and rather more 

 closely arranged toward the front side of the shell, and gradually increase in 

 size and become more distant and oblique posteriorly. They arc only defined 

 on the undulations, and not in the depressions between. 



There are no traces of the lines of growth on any of our specimens ; 

 and the muscular and pallial impressions must be faintly defined, as no indica- 

 tion of them have been observed on any of the casts yet seen. 



The only other species of this type with which I am acquainted is C. 

 bella, described by Mr. Conrad from the Cretaceous rocks of North Carolina 

 (Kerr's Geo]. Report N C, Appendix. 10, pi. 2, figs 2, S), which also occurs 

 in New Jersey, and is very closely allied to the species here described. The 

 only difference I have been able to see is that Mr. Conrad's species has rather 

 more prominent and ventricose beaks, and less closely-arranged radiating 

 markings. In all other respects, the two forms are almost exactly alike. 



Our specimens first described did not show the granules of the surface, 

 but I have since seen them quite clearly on one brought in with more recently 

 received collections. They seem to be arranged in the usual radiating rows. 



Locality and position. — Mouth of Judith River, on the Upper Missouri, 

 in Idaho Territory; in Cretaceous beds now known to hold a position at the 

 horizon of the top of the Fox Hills group. 



Genus NEJERA, Gray. 



Synon. — Necera, Gray (1834), Griffith's An. Kin^d. — Forbes (1843), Proceed. Zool. »Soc, 75; Report Brit. 



Mus. (1«44), 14:3.— Morris (1643), Brit. Foss., 93.— Recluz (1844), Rev. Zool., IV, 47.— 



Hinds (1844) Zool. Snlph., 69. 

 Xeara, Gray (typ. err.), (1839), Rev. Zool., 1-22; (1842), Synon. Brit. Mus., 78; (1847), Z06I. 



Proceed., 92. 

 Cv&pidaria, Nordo (,1840), Rev. Zool., 30. 

 Naeara and Naa-a, Sowerby, (1842), Conch. Man., 95, 198, 310. 



Ktym. — Nrejpa, a nymph, mother of Lampet'a. 

 Type. — ilyu rostrata, Hinds. 



Shell transversely globose-pyriform, distinctly rostrate, and more or less 

 gaping behind ; right valve usually smaller than the other; surface smooth, 

 or concentrically costate or striate ; umbones strengthened within by a pos- 

 terior ridge; hinge provided with a small, oblique, spoon-shaped tooth, or 

 cartilage-process, in each valve, in front of which there is often a small tooth; 

 right valve with a, more or less developed posterior lateral tooth; ligament 



