coNcniFEiiA. 443 



occur on tlie rim of the sliell, and not on the disk, as in Crania* 

 The chief peculiarity of the Ilippuritidce is the dissimilarity in 

 the 'structure of the yalves, but even this is deprived of much 

 significance by its inconstancy, f The free valve of Hippurites 

 is perforated by radiating canals which open round its inner 

 margin, and communicate with 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 in Caprotina, Those which exist in the upper valve 

 of Caprina, and in both valves of Caprinella, have no commu- 

 nication 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., Eig. 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 bivalves, they opened to a very small extent. 



Hippurites, Lamarck. 



Name, adopted from old writers, " fossil Hippuris,''^ or 

 Horse-tail. 



Types, H. hi-oculatus, Lamarck, and H. cornu-vacdnum^ 

 Fig. 237. 



Shell very inequivalve, inversely conical, or elongated and 

 cylindrical ; JixeU valve striated or smooth, with three parallel 

 furrows {I, m, n) on the cardinal side, indicating duplicatures 

 of the outer shell layer; internal margin slightly plaited; pallial 

 line continuous ; umbonal cavity moderately deep, ligamental 

 inflection (Z) 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 ) 



* M. D'Orbigny considers they were produced by peculiar appendages to the 

 mantle-margin, which, in Hippurites, were prolonged into the canals of the upper 

 valve. 



t The lower valves of some Spondyli are squamous or spiny, the upper plain ; those 

 of many oysters, Pectens, and some Tellens are diversely sculptured ; but in no instance 

 is the internal structure of the two valves different. The inconstancy of the shell 

 structure in the Rudistes has a parallel in Ehynchonella and Terebratvla (p. 360), and 

 in the condition of the hepatic organ in Tritonia and Dendronotus. 



X The valves of Crania are perforated by branching tubuli, but in that case they pass 

 vertically ihrough every part of the shell. aM all its layers (p. 361). 



