ART. 4 CAMBKIAN CONCHOSTRACA — ULRICH AND BASSLER 37 



WALCOTTELLA LEPERDITOIDES, new species 



Plate 4, Figure 8 



Description. — Carapace leperditoid in shape, the outline with respect 

 to the long and straight hinge being somewhat oblique, the cardinal 

 angles, especially the anterior, well defined, the upper part of the end 

 margins straight, and the ventral half of the outline semiovate. 

 The surface of the valves rises to a subcentral, broadly rounded apex, 

 the slope toward the ends nearly uniform and toward the dorsal and 

 ventral edges slightly convex. A slight flattening, barely visible even 

 in favorable lighting, occurs along the free margins. Excepting a 

 considerable space about the apex, the surface of the shell is distinctly 

 puncto-reticulate. 



Dimensions of a good left valve: Greatest length 6.2 mm., greatest 

 height 4.2 mm., greatest thickness about 1.9 mm. 



Reinarks. — Were it not for the white, phosphatic shell of this fossil, 

 it might be difficult to remove the impression of close relationship to 

 Leperdiiia that is at once suggested by its general aspect. This 

 impression, however, is quickly invalidated by the shell structure, 

 which is exactly the same as in the other species of Walcottella and 

 contrasts stronglj^^ in color, probable chemical composition, and minute 

 surface markings, wdth that prevailing among the true Leperditiidae 

 from their first appearance in the Beekmantown to the last at the 

 close of the Silurian. That it is not a Leperditia is further indicated 

 by the free edges which surely never overlapped in the ventral part. 

 In every essential feature, except the median tubercle, which is 

 practically obsolete, W. leperditoides agrees with W. pulchella. and 

 that species is most certainly not a close ally of the much later Leper- 

 ditiidae. That the latter were derived out of Walcottella is thought 

 highly probable, but the differences between the known forms of the 

 two types are at least of family importance. 



Compared with other species of WalcoUella, the obsolescence of 

 the median tubercle serves readily in distinguishing W. leperditoides 

 from those simulating it in other respects. 



Occurrence. — Same as the preceding. 



Holotype.—Csit. No. 56484, U.S.N.M. 



WALCOTTELLA NITIDA, new species 



Plate 4, Figures 9, 10 



Description. — Carapace broadly subovate, hinge long, straight, the 

 cardinal angles clearly defined, the anterior one especially sharp; ends 

 not very unequal, the posterior slightly the wider and more uniformly 

 curved at the margin than the anterior, the front outline slightly con- 

 cave in the upper third ; ventral half of outline a nearly regular though 

 broad semiellipse. The right valve figured on Plate 4 is the only 



