204 HIPPURITIDA. 
furrows on the cardinal side, indicating duplicatures of the outer 
shell-layer; internal margin slightly plaited; umbonal cavity 
moderately deep, ligamental inflection with a small cartilage-pit 
on each side; dental sockets subcentral, divided by an obsolete 
tooth; anterior muscular impression elongated, double, posterior 
small, very deep, bounded by the second duplicature; third 
duplicature projecting into the umbonal cavity; free valve 
depressed, with a central umbo, and two grooves or pits corres- 
ponding to the posterior ridges in the lower valve; surface 
porous, the pores leading to canals in the outer shell-layer, 
which open round the pallial line upon the inner margin; ante- 
rior cartilage-pit deep and conical, posterior shallow ; umbonal 
cavity turned to the front; teeth two, straight, subcentral, the 
anterior largest, each supporting a crooked muscular apophysis, 
the first broad, the hinder prominent, tooth-like ; inflections sur- 
rounded by deep channels. 
H. cornu-vaccinum (exvii, 18,19; exviii, 24) attains a length of 
more than a foot, and is curved like a cow’s horn; the outer layer 
‘separates readily from the core, which is furrowed longitudi- 
mally. The ligamental inflection is very deep and narrow, and 
the anterior tooth farther removed from the side than in ZH. biocu- 
Jatus and radiosus (exviii, 25, 26); the posterior apophysis does 
not nearly fill the corresponding cavity in the lower valve. In H. 
-bioculatus and some other species there is no ligamental ridge 
inside; these, when they have lost their inner layer, present a 
cylindrical cavity, with parallel ridges extending down one side. 
‘The third inflection is possibly a siphonal fold, such as exists 
in the tube of Teredo, and sometimes in the valves of Pholas, 
Clavagella, and the caudate species of Trigonia. 
The development of processes from the upper valve, for the 
attachment of the adductor muscles, harmonizes with the other 
peculiarities of Hippurites. The equal growth of the margins 
of the valves produces central umbones, and necessitates an 
internal cartilage; this again causes the removal of the teeth 
and adductors farther from the hinge-margin, to a position in 
which the muscles must have been unusually long, unless sup- 
ported in the manner described. Supposing the animal to have 
had a small foot, like Chama, the mantle-opening for that organ 
would have been completely obstructed by the adductor, but 
that the muscular support was hook-shaped. The posterior 
-adductor-process is similarly under-cut for the passage of the 
rectum, which in all bivalves emerges between the hinge and 
posterior adductor, winds round outside that muscle, and termi- 
nates in the line of the exhalent current. There is a groove 
(sometimes an inch deep) round the second and third duplica- 
tures in the upper valve, which seems intended to facilitate the 
passage of the alimentary canal, and the flow of water from the 
