850 



NATURAL HISTORY. 



Genus Myadora. In Myadora the valves are exactly the converse of Pandora, the left beino- 

 flat and the right convex. The outline is more triangular than that of Pandora, and there is a 

 free sickle-shaped ossicle in the right valve and two tooth-like ridges in the left. Ten species 

 are found living in New Zealand, New South Wales, and the Philippines. 



Genus Myocltama. In Myochama the animal is attached by the right valve, while the 

 left is round ; the cartilage is internal ; there are two tooth-like projections in. each valve. 

 It is attached to Trigonice and Crassatellce. Its habitat is New South Wales. 



Genus Chamoslrea. The shell is solid, and attached by the front side of the right valve, 

 which is deep and strongly keeled. One species only is known, from New South Wales. 



FAMILY XXII. GASTROCELEXID2E. 



They have thin, gaping, toothless valves, united by a ligament, and cemented to a shelly tube 

 when adult. The animal has two very long united siphons behind, and a truncated finger-like foot 

 in front. The members of this family are burrowers, either in mud or stone, near low water. 



Genus Gastrochcena.* The shell is wedge-shaped, the umbones are turned forward, the 

 valves gape widely in front and are closed behind. Gastrochcena perforates shells and limestone; 

 its holes are regular, about two inches deep ; the external orifice is hour-glass-shaped and lined with 

 shell. Ten species are known in the West Indies, Britain, Red Sea, Pacific Isles, Panama, &c. 



Genus Saxicava. The young shell is said to be symmetrical and furnished 

 with two teeth in each valve ; but the adult is rugose, toothless, thick, oblong, 

 gaping, with an external hinge ligament; the siphons are large and united near the 

 ends. So variable is this shell that five genera and fifteen species have been named 

 upon its aberrant forms. It conceals itself in the crevices of rocks and coral and 

 amongst the roots of seaweed. At Harwich it bores into the Clay ironstone, at 

 Folkestone into the Kentish Rag, and at Portland into the Portland Oolite. Its 

 crypts are six inches long. Saxicava ranges from low water to 140 fathoms ; it is 

 found in all Arctic seas. Specimens of Saxicava arctica were more abundant than 

 any other shells brought home by the Alert and Discovery from the Arctic regions. 

 Among this section of mollusca are some instances which present the phenomenon of 

 an extensive geographical distribution, though their capabilities for locomotion are 

 very limited. For instance, some species of Saxicava arctica, Venus pullastra, and 

 Pecten pusio are found both on northern shores and at the Cape of Good Hope, 

 though not in the intermediate tropical regions. The species of Limacina which 

 belongs to the South Polar Ocean cannot be distinguished from the Limacina arctica 

 belonging to the North ; it has no representative in the intermediate seas. The 

 same is the case with the genus of Puncturella, which embraces two species, of which 

 the one belongs to the Arctic, the other to the Antarctic Seas, in the neighbourhood 

 of Tierra del Fuego. 



Genus Clavagella. In this genus the shell is oblong, irregular; the valves 

 unequal, the right valve always free, the left embedded in the dilated hind part 

 of the tube, which is shelly, cylindrical, attenuated, and open behind ; the margin 

 is simple or furnished with siphonal fringes. The anterior or lower end of the tube 

 is club-shaped and either simple or surrounded by spine-like tubes ; the mantle 

 being furnished with tentacular processes forms these branching tubuli. Most 

 of the Clavagellce burrow in stone and coral. Six species occur in the Mediterranean, 

 Australia, and the Pacific. 



Genus AspergiUum, " Watering-pot Shell." In certain boring and burrowing bi- 

 ASPERGILLUM. valveSj as Gastrochcena, Clavagella, and Teredo, the shell does not increase with age, but 

 the siphons secrete a shelly tube in which the soft parts of the animal are encased, and the minute valves 

 of the young mollusc are seen embedded in the wall. In AspergiUum vaginiferum, the Watering-pot 

 Shell, the minute valves are also to be seen near the lower extremity of the tube, the siphonal end 

 being plain, or ornamented with from one to eight frills. Twenty-one species occur in the Red Sea, 



Java, New Zealand, &c. 



* Greek, gaster, belly, and chana, gape. 



THE TVATEKIXG- 

 POT SHELL, 



