628 MOLL use A - CLASS III. —SCAPIIOPGDA. 



whilst the left 13 convex ; the shell is also very inequilateral. There are 

 two diverging resilium grooves at the hinge. It lives buried in sand. 



The single species Ckamostren albida, found off the coast of New South 

 Wales, constitutes the family CHAMoSTRiEiDiE. It has a thick, pearly shell ; 

 the right valve, by which it is attached, is deep and strongly keeled ; the left 

 valve ia flat, with a conical tooth in front of the resilifer; the ossicle is 

 extremely large. 



The Vebiicordiid^ and Lyonsiidje are deep-water forms not often met 

 with. 



The Anatinib^e have extremely thin shells, sometimes granular on the 

 surface, and gaping behind. The genus Thrcwia is British. 



The Pholadomyip^, represented to-day by a single species, Pholadomya 

 Candida, in the West Indies, was formerly an abundant group. The shell is 

 oblong and equivalve, gapes behind, and is ornamented with obscure radiat- 

 ing ribs. The ligament is internal, and there is one tooth in each valve. 



The Clavigellid^e are in some respects the most interesting members of 

 the group. One genus, Clamgdla, bores into rocks, the left valve becomes 

 attached to the wall of the burrow, the right remaining free ; the passage to 

 the exterior, occupied by the siphons, has a shelly lining, which projects 

 freely on the exterior and terminates in two or more frills. 



The other principal genus Brechites [ = Aspergillum], known as the 

 watering-pot shell, burrows in sand, lining the burrow with a shelly tube, 

 into the wall of which the valves of the young shell are built. The free end 

 of the tube is also frilled. The adductor muscles not being required are 

 aborted. When the animal is full grown the bottom of the tube is closed 

 with the familiar perforated plate, secreted by the fringed foot. It occurs 

 only in the tropical seas. 



Order V. — Septibranchiata 

 Contains two families — Poromyid^ and CasPlDARiiD^ — principally inter- 

 esting on account of the modification of the gills to form a transverse septem 

 as already described (ante, p. 620). The Poromyid^ are rounded shells with 

 a single tooth in each valve. The siphons are fringed. The Cdspidariid^ 

 are represented in British waters. The shell is beaked behind, and the 

 valves are closed except at the end of the beak through which the fringed 

 siphons protrude. There are no teeth in the hinge. 



CLASS III.— SCAPHOPODA, or SOLENOCONCHA. 



The Elephant's-tooth shells belong to a small but very distinct class of 

 low organisation. The shell is long, slightly curved, and tapers almost to a 

 point, resembling in external appearance a miniature elephant's tusk. It is, 

 of course, quite hollow, and there is an opening at each end ; the one at the 

 small end is generally notched on the ventral, i.e., the convex side. Some- 

 times the shell is finely striated lengthwise. The animal is symmetrical. 

 The margins of the mantle are united beneath, forming a long tube open at 

 either end ; a barrel-shaped prominence or snout represents the head, and 

 the mouth at its extremity is surrounded by leaf like tentacles. 



The formula of the radula is llll'l. The alimentary canal is 

 doubled on itself, and terminates about the middle of the ventral side, 

 opening into the mantle cavity, which latter has its efferent opening at the 

 apex of the shell. There are no gills properly so called, but their function 



