40 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



while in the matter of the reduction of the median tubercle, it is no 

 worse off than W. leperditoides. 



Occurrence. — Same as W. apicalis, W. leperditoides, etc., Bright 

 Angel shale, Grand Can3^on, Ariz. 



Holotype.—C&t. No. 56487, U.S.N.M. 



WALCOTTELLA LIMATULA, new species 



Plate 5, Figures 1-3 



Description. — Carapace subovate, wide posteriorly, narrow ante- 

 riorly, with a long straight hinge line and obtusely angular cardinal 

 extremities; anterior margin most prominent in the upper third 

 beneath which it recedes rapidlj^ in the lower two-thirds; ventral 

 margin rather strongly convex, its posterior third, together with the 

 whole of the posterior outline to the extremity of the hinge, forming 

 a uniform circular curve. Valves only moderately convex, the curva- 

 ture of the surface, disregarding the nodes, almost uniform from end 

 to end; free margins, when perfect, with a distinct, flattened border. 

 When this border is broken away, as in Figure 1, on Plate 5, the form 

 appears more elongate than normal. Main tubercle located about 

 one-fifth of the length in advance, and slightlj^ above the middle, 

 strong, conical, conspicuously elevated above the general surface of 

 (he valve. Midway between this tubercle and the anterior extremity 

 of the hinge is a much smaller, ocular node. Surface of shell very 

 finely puncto-reticulate, the pattern smaller than in any of the pre- 

 ceding species except W. pulchella. 



Dimensions of the type specimen: Greatest length 5.0 mm.; 

 greatest height 3.2 mm., greatest thiclcness (between apices of main 

 tubercles) 1.8 mm. 



Remarks. — This seems to be the commonest of the species of the 

 genus collected by Walcott at his localit}^ in Bass Canyon, nine or 

 ten specimens being confidently referred to it. While the species 

 appears to be fairly constant, it may yet be observed that in some of 

 the specimens the ventral part of the outline is less convex, the ante- 

 rior extremity less narrowly curved, and the main tubercle a little 

 stronger than in Figure 2 on Plate 5. Such a specimen is represented 

 by the original of Figure 1 on the same plate. Possibly they consti- 

 tute a recognizable variety. 



Compared with other species of the genus, W. limatula is easily dis- 

 tinguished by the shape of its valves. Of those approaching it most 

 in this respect, W. ventrosa will be found to be shorter and to have the 

 ends more nearly equal in size and curvature of outline. W. suhtrun- 

 cata and W. longula also are closely allied but differ so obviously from 

 W. limatula in the truncation and greater width of their anterior 

 extremities, that comparison between them is not likely to occur. 



Occurrence. — Bright Angel Shale, Grand Canyon, Ariz. 



Cotypes. —Csit. No. 56488, U.S.N.M. 



