118 LECTURES ON MOLLUSCA. 



Modiolarca has a thin shell moored to floating sea-weed, and greatly 

 resembles Modiolopsis in shape. This also has the mantle-flaps united. 

 Leiosolenus represents Litliopliagus in this family, from which the 

 shell alone cannot he distinguished. It has however siphon pipes, and 

 excavates a deep and very spacious burrow, like Gastrochcena. 



The next group of families differ in the same way, as to the posses- 

 sion or absence of siphon pipes. They agree in having the foot large, 

 bent, and deeply grooved ; and in having numerous teeth at the hinge. 



Family ARCADE. (Arks.) 



The boat-shaped Arks are easily known by their distant umbos, with 

 straight hinge and two well-marked muscular impressions. The 

 mantle is freely open, without pipes, and the mouth is not provided 

 with lips. The hinge may be regarded as having two diverging teeth, 

 each of which is cut across into numerous smaller ones. In old speci- 

 mens these are often obsolete, and a ridge appears instead. In Area 

 proper, the shell is cockle-shaped, and lives freely in sand or mud, 

 crawling on its crenated foot. In Scapharca, which abounds on the 

 shores of the southern States, the valves are unequal, and generally 

 thin. The American genus Noetia is like an ark with one side cut off. 

 Argina, also an American form, is more regular ; but with one row of 

 hinge teeth very short and twisted. In Lunarca, which closely re- 

 sembles it in form, the short tooth is not serrated. Trisis has the valves 

 shaped like Byssoarca, but curiously twisted. It has some resemblance 

 to the curious little fresh-water Ark, Scaphula, from the East Indian 

 rivers, in which however the teeth are rather transverse at the ends, 

 forming a transition to Cucullcea. In this group the serrations of the 

 teeth are normal in the middle, but parallel to the hinge line at the 

 ends. The posterior muscular scar is bounded by a stout ridge. This 

 form is now almost extinct, but in the oolitic and cretaceous strata it 

 was very abundant. In the Macrodon group of the older rocks, only 

 the shorter hinge tooth is serrated, the longer one remaining as in 

 Unio. 



One large group of Arks is completely sedentary in its habits, re- 

 maining fixed in crevices or old burrows. But instead of spinning a 

 byssus like the Mussels and Pinnas, it adheres by the end of its foot, 

 which deposits a number of horny plates, which can be cast off and 

 renewed on special occasions. It appears more convenient to regard 

 Cockle-arks (A. grandis, &c.) as the types of the family, and to call 

 the fixed species Byssoarca. The typical forms have long straight 

 hinges, winged on each side, with very numerous sharp teeth, and a 

 gape in front where the creature fastens itself, with its face to the 

 corner like a naughty boy. In the common form Barbatia, the wings 

 are rounded off, the gape is not seen, and the hinge line is slightly 

 curved, forming a transition to Pectunculus. 



Fossil Arks are found in great numbers in every age, the pala3ozoic 

 forms being chiefly of the Cucullcea, Cucullella, and Isoarca type. 

 They live now at all depths, from low water to two hundred and 

 thirty fathoms ; and in all climates, from the equator to Prince Kegent 



