38 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 87 



specimen seen. It is clean cut but largely exfoliated and retains 

 small patches of the delicate white exterior layer of the shell only 

 along the margins. Except along the anterior side there is no evi- 

 dence of a flattened border, but on the internal cast an impressed line 

 follows the ventral edge and marks oil" a narrow band. The surface 

 is rather strongly convex, reaching its greatest elevation a short dis- 

 tance in advance of the middle of the valve in a low, conical, radially 

 marked eminence representing the median tubercle of the other species 

 of the genus. Between this elevation and the antero-dorsal angle is 

 a smaller but more sharply defined node regarded as the ocular 

 tubercle. Just in front of this and extending dowTiward parallel with 

 the anterior edge, a shallow furrow. Surface of shell puncto-reticu- 

 late, the pattern and size of punctae about as in W. breniuscula, 

 figured on the same plate. The punctae extend through the outer 

 into the inner layer of the shell. 



Dimensions of the type specimen, a right valve: Length 4.8 mm., 

 greatest height (across the posterior half) 3.2 mm., greatest thickness 

 about 1.3 mm. 



Remarks. — The general aspect of this valve is highly suggestive of 

 certain species of Isochilina, an Ordovician and Silurian genus of 

 Leperditiidae. However, for essentially the same reasons mentioned 

 in discussing the resemblance of W. leperditoides to Leperditia, the 

 evidence is convincing that in this case also the resemblance is not 

 based on true affinity. An ocular node and the shallow anterior 

 furrow are both present in W. apicalis, but the median tubercle is 

 so much stronger in that species that, while their generic alliance seems 

 assured, their specific relations can not be very intimate. Both W. 

 pulchella and W. leperditoides probably are nearer allies, but neither 

 of these has an ocular tubercle. 



Occurrence. — Sandstone bed in Bright Angel shale, Bass Canyon, 

 Grand Canj'on of the Colorado, Ariz. 



Holotype^.—C&t. No. 56485, U.S.N.M. 



WALCOTTELLA OBLONGA, new species 



Plate 5, Figure 8 



Description. — A right valve only has been observed, and this is 

 imperfect in the posterior and ventral half of the outline. So far as 

 preserved and interpreted, the carapace is oblong or subcylindrical 

 mth rounded ends, the hinge line straight, long, the anterior angle 

 obtuse but well defined, the posterior angle more indefinite, the 

 ventral side gently convex and approximately parallel to the hinge. 

 Surface rather strongly convex, the median tubercle not very con- 

 spicuous and situated its diameter in advance of the middle of the 

 valve. The eye node is barely distinguishable and the puncto- 



