204 HIPPURITIDiE. 



furrows on the cardinal side, indicating duplicatnres 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 grooA^es 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 apophj^sis, 

 the first broad, the hinder prominent, tooth-like ; inflections sur- 

 rounded by deep channels. 



H. cornu-vaccivum (cxvii, 18, 19 ; cxviii, 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- 

 nally. The ligamental inflection is very deep and narrow, and 

 the anterior tooth farther removed from the side than in H. biocu- 

 latus and radwsus (cxviii, 25, 26) ; the posterior apophj^sis 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 thej'^ 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 



