Il8 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



The larger shells are almost squarely truncated at tlie ventral margin, the 

 keel often being very slight, though always visible. The sides are flat- 

 tened or gently convex, and ornamented with about forty-five to fifty ribs 

 counted along the ventral margin, where they terminate in flat, transverse 

 tubercules. On the umbilical margin of the whorl there is a prominent row 

 of tubercules not shown in Gabb's figure, though mentioned in the text, from 

 which the umbilical wall makes a perpendicular descent. The costal nodes 

 are not always very conspicuous upon the casts, though three or four rows 

 can be distinctly made out. 



The young shells of this species, unlike those of Trask's species, are 

 almost perfectly smooth, showing neither ribs nor costal nodes until they 

 attain a diameter of more than 2 cm. Gabb seemed lo have noticed this 

 fact, though without attaching to it the importance which it deserves. The 

 specimen figured by Trask had a diameter of 1.5 cm., yet distinctly showed 

 twenty-four strong ribs. In the young shell of Gabb's species the umbilicus 

 has a diameter of less than one-fourth that of the entire coil, and the section 

 of the whorl is narrow and elongated, and rather squarely truncated on both 

 dorsal an<l xenlral margins. 



The largest specimens of S. gabbi in the collections of the University of 

 California have a diameter of 10 cm., and at that size the ribs have almost 

 disappeared, together with the nodes upon the sides of the sliell. 



The above name is proposed to distinguish this species 

 from that for which it has evidently been mistaken. There 

 are some varieties of the species that deserve mention, one, 

 especially, in which the sides are more than ordinarily 

 convex, and in which the ventral truncation is somewhat 

 rounded. 



64. Schlcenbachia buttensis, sp. nov. 



Plate IV, Figs, no, in. 



This species is related to S. gabbi, though it is evidently a distinct form. 

 The ribs, about fourteen in number, counted along the umbilical shoulders, 

 .are nodose and bifurcating. The nodes are in five rows upon the sides of 

 the shell, and in this respect it resembles its congener, .S. gabbi, but the 

 umbilical row is much more elevated and narrow, and the ribs are more 

 disposed to bifurcate. This takes place from either of the three inner rows 

 of nodes. The nodes of the outer row are sharp and ridge-like, forming 

 upon the periphery a flattened, ventral surface, as shown upon the cast. The 

 keel is low and apparently entire; septation not well known. 5". buttensis 

 is also related to S. varians Sowerby. 



The figure was drawn from an imperfect specimen, 

 immature in size, 3'^et sufficiently large to show the specific 

 characters. 



