CONCHIFERA. 443 
occur on the rim of the shell, and not on the disk, as in Craniu.* 
The chief peculiarity of the Hippuritide is the dissimilarity in 
the structure of the valves, but even this is deprived of much 
significance by its inconstancy.t 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 Caprinelia, 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 (Pl. 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 hgament appears to haye 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. bi-oculatus, Lamarck, and H. cornu-vaccinum, 
Fig. 237. 
Shell very inequivalye, inversely conical, or elongated and 
cylindrical; fixed valve striated or smooth, with three parallel 
furrows (/, m, n) on the cardinal side, indicating duplicatures 
of the outer shell layer; internal margin slightly plaited; pallial 
line continuous ; umbonal cavity moderately deep, igamental 
*nuflection (7) with a small cartilage-pit on each side (c,c); dentai 
sockets sub-central, divided by an obsolete tooth; anterior 
inuscular impression (a) elongated, double; posterior (a) 
* MM. D’Orbigny considers they were produced by pectlizr a;pendages to the 
mantle-margin, which, in Aippurites, were prolonged into the cana‘s of the upper 
vaive. 
+ The lower valves of some Spondyli are squamous or spiny, the upper plain; those 
of many cysters, Fectens, andsome Tellens are diversely sculptured ; but in no instance 
is the internal structure of the two valves different. The inconstancy of the shell 
strocture in the Rudistes has a parallel in Ahynchonella and Terebratula (p. 360), and 
ii) tne condition of the hepatic organ in Zritonia and Dendronotus. 
t Lhe valves of Crania are ; er{crated by branching tubuli, but in that case they pase 
esrtiea I through every part of the shell, am allits layers (p, 361). 
