BIVALVES VENUS SHELLS, COCKLES, ETC. 625 



behind. The hinge plate is broader and there are no lateral teeth. As their 

 name indicates, they are sand dwellers. A few occur on the British coasts, 

 but the finer examples come from the Pacific and Indian Oceans. 



The DONACID^E, or wedge shells, are very inequilateral, the front portion 

 being much longer than the truncated hinder part. The exterior is usually 

 smooth, the umbones are directed backwards, and the margins of the valves' 

 are usually very finely crenulated, whilst the pallial sinus is deep. There 

 are two or three cardinals and two laterals, one anterior and one posterior, 

 in each valve. 



The TANCREDIID^E have very similar shells to the Donacidse. 



The MACTRID^E are a large and widely- distributed family dwelling in sand. 

 The shell is equivalve and somewhat triangular ; commonly smooth and white, 

 with a trace of periostracum. The hinge is usually marked by the conspicu- 

 ous internal resilium lodged in a deep pit ; but in some forms the whole 

 ligament is external. The hinge-plate is well developed; the teeth vary 

 somewhat, typically there is a A-shaped cardinal tooth in each valve in front 

 of the resilifer, and single anterior and posterior lateral teeth in the right 

 valve fitting into double ones in the right one. 



The CARDILIID^: are very similar in their shell to the preceding, and are 

 probably closely allied to them, bub their anatomy is still unknown. 



The MESODESMATIDJK resemble the Mactridae in their anatomy, but the 

 siphons are not united, the shell usually is more like that of Donax with 

 umbones directed backwards. 



Sub-order 3 VENERACEA. These have usually short siphons, and there- 

 fore a moderate pallial sinus. The shells of 

 the principal family, VENERID^E, are remarkable 

 for the elegance of their forms and colours. 

 Their texture is very hard and procellaneous, 

 the exterior is frequently smooth and highly 

 polished, sometimes ornamented with V-shaped 

 lines of colour,sometimes with concentric grooves, 

 \vh : lst a few have projecting plates or spines 

 (Fig. 10). The margins are closed, and often 

 crenulate. The siphons are more or less united. 

 The ligament is external, and there is a well- Fig. 10. CYTHERKA DICNK. 

 marked lunule ; the hinge is strong, and con- 

 sists of three well-developed cardinal teeth in each valve, two out of the 

 three appearing double ; the laterals are variable. 



The PETRICOLID^E, which occupy burrows in limestone, are very elongate ; 

 in their shape and external markings they closely resemble the well-known 

 rock-borer Pholas, though they are readily distinguished by the possession 

 of three cardinal teeth in each valve. 



The GLAUCOMYID^E, which are fresh, or brackish-water forms, have very 

 long siphons, and live buried in the mud of estuaries in India, China, and 

 the Eastern Archipelago. The shell is shaped like Mya. 



Sub-order 4. CARDIACEA comprises the cockles and their allies. The shell 



is heart-shaped, with an internal ligament, and there are She or two cardinals 



in each valve ; whilst the right valve usually carries two anterior and two 



posterior laterals, which receive the corresponding single teeth of the 



41 



