CONCHIFERA. 283 



lonal cavity : free valve depressed, with a central umbo, and two grooves 

 >r pits corresponding to the posterior ridges'in the lower valve; surface 

 orous, the pores leading to canals in the outer shell -layer, which open round 

 he pallial line upon the inner margin ; anterior cartilage-pit deep and conical, 



Fig. 196. H. Toucasianus, upper valve, $.* Fig. 197. Lower valve, with mould, . 



I, ligamental ; m, muscular; n, siphonal inflections; a;, fracture, showing canals; 

 ?, cartilage: , left umbo; the arrows indicate the probable direction of thebranchial 

 currents. 



posterior shallow ; umbonal cavity turned to the front (u} ; teeth 2, straight, 

 sub-central, the anterior largest, each supporting a crooked muscular apo- 

 physis, the first broad, the hinder prominent, tooth -like ; inflections (m, n) 

 surrounded by deep channels. 



H. comu-vactinum 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 longitudinally, The ligamental inflection (I) is very deep and 

 narrow, and the anterior tooth further removed from the side than in H. 

 bi-oculatus and radiosus (figs. 194, 5) ; the posterior apophysis (#') does not 

 nearly fill the corresponding cavity in the lower valve. In H. bi-oculatus 

 and some other species there is no ligamental ridge inside ; these, when they 

 have lost their inner layer, present a cylindrical cavity with two parallel 

 ridges, extending down one side. The third inflection (n) 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 the Hip- 

 purite. The equal growth of the margins of the valves produces central 

 umbones, and necessitates an internal cartilage; this again causes the removal 



* This internal mould, representing the form of the animal, was obtained by re- 

 moving the upper valve piecemeal with the chisel ; a plaster-cast taken from it 

 represents the interior of the upper valve, with the bases of the teeth and apophyses. 

 See originals in Brit. Mus. 



