94 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 78 



to point out satisfactory differences. The carapace of Schizodiscus, 

 however, is supposed to be peltate, while in Lepiditta the two valves 

 are thought to have closed as in Estheria. Compared further with 

 the latter genus Lepiditta differs in the form of its valves, these being 

 much narrower and subacuminate instead of rounded, posteriorly; 

 and the concentric markings of the shell are less regular and not 

 ridgelike. The small interior clavicle in front of the umbones is an 

 interesting reminder of a similar feature in Ribeiria, Ischyrina, and 

 Technophorus. 



Only two of the four species referred to this genus by Matthew are 

 regarded as strictly belonging here. These are his L. alata and 

 L. curta. The other two, L. auriculata and L. sigillata, are each rep- 

 resented by a single example that in neither case is sufficiently good 

 to encourage an attempt at final classification. Provisionally they 

 must be viewed as doubtful species. 



A possible third good species is indicated by Steusloff 's Bythocypris 

 polita. As figured and described, this species agrees very well with 

 L. alata in general aspect, but differs in its smooth, polished shell and 

 in the unusual abruptness of the ventral slope. As it is most cer- 

 tainly not a Bythocypris, it is removed provisionally to Lepiditta. 



LEPIDITTA ALATA Matthew 



Plate 7, Figures 24, 25 



Lepiditta alata Matthews, Trans. Royal Soc. Canada, vol. 3, sec. 4, 1886, p. 61, 

 pi. 6, figs. 16, 16a; Trans. New York Acad. Sci., vol. 15, 1896, p. 194, pi. 

 15, figs, la, 16. 



Description. — Valves obliquely semielliptical, broad and rectangu- 

 lar on the anterior side, narrowing posteriorly; hinge straight, extend- 

 ing the full length of valves. Surface moderately convex, marked by 

 numerous, rather regular but not very sharply defined concentric 

 lines. These center at the small umbonal elevation which is located 

 just within the dorsal edge and approximately one-third of the length 

 of the hinge behind the anterior angle. Very delicate lines radiating 

 from the umbo observed on a cast of the exterior. In casts of the 

 interior the umbo is more prominent than on the exterior and the 

 clavicle causes a small, sharp, sometimes fissurelike, vertical depres- 

 sion just in front of it. 



Dimensions: Length, 1.15 mm.; greatest height 0.89 mm. 



Remarks. — The above description is based on the specimens selected 

 by Matthew as the types of his species. Except that they do not 

 preserve the test, they are in good condition; and being molds of 

 the two surfaces of the shell, internal characters are seen that are not 

 usually visible. One of the molds of the exterior surface shows two 

 obscure radial ridges disposed somewhat as in the Pennsylvanian 

 Leaia leidyi. Although these obscure radial ridges are to be observed 



