124 PATAGONIAN EXPEDITIONS I PAL/EONTOLOGY. 



Remarks: Differs from C. lyelli in the subtriangular, shorter outline, 

 the concave lunular margin, and the crenulations of the lower margin. 

 As to the latter point, compare remarks above. 



Record of specimens : Mouth of Santa Cruz River, 4 right, 4 left valves. 



Distribution: Santa Cruz, Patagonian formation (v. Ih.). 



Affinities: C. plicatilis Deshayes (1860, p. 745, pi. 18, f. 6, 27) from 

 the Middle Eocene of Paris comes near this species, but C. kokeni is 

 narrower posteriorly, the surface markings are much more distant from 

 each other, and the anterior dorsal margin is concave. Still more closely 

 allied is C. sulcata (Sol.) (see Wood, 1871, p. 170, pi. 23, f. n) from the 

 Upper Eocene of England and France, which agrees especially well in 

 sculpture, while the outline is more like that of C. plicatilis, although 

 more produced posteriorly. 



48. CRASSATELLITES QUARTUS (Ortmann). 



PL XXVII, Fig. i. 

 1900 Crassatella quarta Ortmann, in: Amer. Journ. Sci., v. 10, p. 371. 



Shell ovato-elongate, comparatively thin, less convex than in the pre- 

 ceding species, but more so than in C. lyelli. Apex only slightly promi- 

 nent. Anterior end of shell rounded, posterior hardly angulated and 

 hardly narrowed. Posterior dorsal margin straight near apex, anterior 

 straight, only with a slight suggestion of concavity close to the apex. 

 Surface ornaments as in C. lyelli, but the ridges more crowded, and a 

 little less strongly developed. Ventral margin without crenulations. 



Measurements of Santa Cruz specimen: Length, 25 + x ; height, 15 

 mm; of Lake Pueyrredon specimen: Length, 17 mm; height, 10 mm. 



Remarks: The single isolated valve from Santa Cruz is broken pos- 

 teriorly, so that the complete length cannot be given. According to the 

 lines of growth, however, we have here the following relations : Length, 

 1 6 mm; height, 9 mm, which agrees well with the Lake Pueyrredon 

 specimen, and would bring up the total length of the shell to about 30 

 mm (not quite double its height). 



Record of specimens: Mouth of Santa Cruz River, i left valve; Lake 

 Pueyrredon, 600' above base, i cast of left valve. 



