282 



MANUAL OF THE MOLLUSCA. 



the upper surface by numerous pores, as if to supply the interior with filtered 

 water ; possibly, they were closed by the epidermis.* 



In the closely allied genus Radiolites there is no trace of such canals, nor 

 ia Caprotina. Those which exist in the upper valve of Caprina, and in both 

 valves of Ca'prinella, have no communication with the outer surface of the 

 shell ; they appear to be only of the same character with the tubular ribs of 

 Cardium costatum (PI. XIX. fig. 1), and it is highly improbable that they 

 were permanently occupied by processes from the margin of the mantle. 



The teeth of the left, or upper valve, are so prominent and straight, that 

 its movement must have been nearly vertical, for which purpose the internal 

 ligament appears to have been exactly suited by its position and magnitude ; 

 but it is probable that, like other bi-valves, they opened to a very small extent. 



Fig. 194. Interior of lower valve, |. Fig. 195. Upper valve (restored). 

 Hippurites radlosus, Desm. Lower Chalk, St. Mamest, Dordogne.i 

 a, a, adductor impressions and processes ; c, c, cartilage pits ; t, t\ teeth and dental 

 sockets ; u, umbonal cavity ; p, orifices of canals ; I, ligamental inflection ; m, mus- 

 cular; n, siphonal inflection. 



HirruRiTES, Lamarck. 

 Name, adopted from old writers, " fossil Hippuris" or Horse-tail. 

 Tz/pes, H. bi-oculatiis. Lam. and E. cornw-vaccinum, fig. 198. 

 jS)^^// very inequivalye, inversely conical, or elongated and cylindrical; 

 fxed valve striated or smooth, with three parallel furrows (/, m, n,) on the 

 cardinal side, indicating duplicatures of the outer shell layer : iutemal margin 

 slightly plaited; pallial line continuous; umbonal cavity moderately deep, 

 ligamental inflection {I) with a small cartilage-pit on each side {c, c); 

 dental sockets sub-central, divided by an obsolete tooth ; anterior muscular 

 impression {a) elongated, double ; posterior (a') small, very deep, bounded by 

 the second duplicatm-e {m) ; third duplicatm-e {n) projecting into the um- 



* The A'alves of Crania are perforated by branching tubuli, but in that case they 

 pass vertically through every part of the shell, and all its layers (p. 214.) 



+ From the original in the Brit. M. The inner layer of shell In this species has 

 ^n irregularly cellular structure, to which its preservation is due. 



