MANUAL OF THE MOLLUSCA. 



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

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



In the closely allied genus Radiolites there is no trace of such canals, no 

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

 valves of Caprinella, have no communication with the outer surface of th 

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

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

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



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

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

 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 exten 



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



Hippurites radiosus, Desm. Lower Chalk, St. Mamest, DordogneA 



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



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



cular; n> siphonal inflection. 



HTPPURITES, Lamarck. 



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



Types, H. bi-ocidatus, Lam. and R. cornu-vaccinum, fig. 198. 



Shell very inequivalve, inversely conical, or elongated and cylindrical ; 

 fixed valve striated or smooth, with three paraEel furrows (I, m, n,} on the 

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

 slightly plaited; pallia! line continuous; urabonal cavity moderately deep, 

 ligamental inflection (1} with a small cartilage-pit on each side (c, <?); 

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

 impression (a) elongated, double ; posterior (a 5 ) small, very deep, bounded by 

 the second duplicature (m) ; third duplicature (n) projecting into the um- 



* The valves 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.) 



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

 an irregularly cellular structure, to which its preservation is due. 



