126 University of Texas Bulletin 



and the ventral margin less nearly circular. 1 I know of no other similar 

 species from the Comanchean. 



Shell oblique, elliptical in outline except for the projecting, elevated 

 umbonal region. Beaks greatly recurved. Ventral margin almost an el- 

 lipse, sharply curved below the beak, gently curved behind ; dorsal-posterior 

 margin nearly straight. The valves have about 24 sharply raised ribs, 

 which bear remote but unevenly spaced, elevated, nodose imbrications. 

 The lateral dimension of the shell is thick, and the species varies some- 

 what in outline. The hinge and other interior details are unknown. 



PROTOCARDIA p. aff. MULTISTRIATA (Shumard) 

 PI. 10, figs. 21-26, 32 



This beautiful Protocardia occurs as casts and molds in the clay-iron- 

 stone of the Gainesville brickyards and shows in nacreous preservation 

 the minutest details of the exterior and the interior of the shell. Most 

 frequently the shell has crumbled into a friable powder and falls to pieces 

 as the rock is broken open. It then leaves in the ironstone excellent molds 

 of the exterior of the shells showing the radial and concentric striations 

 of the exterior and the details of the hinge, and casts of the interior 

 showing details of the dentition, the two muscle scars, pallial line, etc. 

 Frequently the shell has been removed by weathering and the ironstone 

 carries a > cavity of its exact shape bounded by the cast and the mold. 

 Rarely in the ironstone but usually in the blue shale, the original shell 

 is preserved intact. 



The Protocardia group of the Texas Comanchean has not been satis- 

 factorily studied and its species are still poorly defined. As in many 

 other Texas genera this uncertainty can be resolved only by a critical 

 description and photographic illustration of the types. Here as in other 

 genera where new species might have been founded, this has not been 

 done pending a better understanding of the relations of the species already 

 described. 



Shell inequivalve, inequilateral, biconvex, ornamented with sixteen or 

 more serrate radial lines and about 75 round topped subequal concentric 

 ribs separated by narrow valleys. The dorsal margin of the shell is 

 arcuate interiorly, and anteriorly is more thickened. It bears several im- 

 bricated lamellae parallel to and below the hinge line. The region under 

 the beak is deeply excavated. 



'Harris, The Midway Stage, Bull. Amer. Pal., 4, pi. 4, flg. 12. 1896; and, The Lignitic 

 Stage, Bull. Amer. Pal., 9, pi. 11, fig. 1, 1897. 



