XVIII 



ECARDINES: INTERNAL CHARACTERS 



495 



of the Trimerelliclae. A pair of " transverse '' scars (f) occurs 

 in each valve between the " terminals " and the antero-lateral 

 edge of the "platform" (y). "Cardinal" (v), "sub-cardinal" 

 (w), and " umbo-lateral " (x) scars also occur. The median 

 impression which covers the " platform " (y) consists of a 

 central, lateral, and usually an anterior pair of scars ; and the 

 impressions of the genital organs, according to Davidson and 

 King, lie medianly posterior to the "platform." The "platform" 



Fig. 323. — Trimerella. (After Davidson and King.) A, Inner surface of pedicle-valve 

 or ventral valve: a, pseudo-deltidium ; 6, deltidial slope; c, deltidial ridges; d, 

 areal borders; e, cardinal callosities; /, cardinal facet; g, lozenge; i, umbonal 

 chambers separated by cardinal buttress; j, platform ; k, platform vaults; I, median 

 plate; m, median scars; n, anterior scars; o, lateral scars; p, post-median scars; 

 Q-, crown crescent ; r, side or lateral crescent; .s, end or terminal crescent; «, trans- 

 verse scars; u, archlet (vascular sinuses); vj, sub-cardinal scars; x, umbo-lateral 

 scars. B, Brachial or dorsal valve: e, cardinal sockets; j, platform ; k, platform 

 vaults; /, median plate ; ?>i, median scars ; ;^, anterior scars; g, crown crescent ; 

 r, side or lateral crescent; s, end or terminal crescent; t, transverse scai-s; 

 u, archlet (vascular sinuses); v, cardinal scars; to, sub-cardinal scars. 



itself is a more or less consj^icuous central calcareous elevated 

 area occurring in each valve, but most developed in the dorsal ; 

 in some cases it is double-chambered with tubular cavities 

 (" platform vaults," Fig. 323, A, B, A:), in others it is more or 

 less solid. It appears to have originated through a posterior 

 shifting of the central muscular bands, that they might be inserted 

 behind the liver; at the same time a deposition of shelly material, 

 to form fulcra to work the heavy valves, took j)lace at these 

 points. The tunnelling-out of the platform was probably due 



