278 



LOWER INVERTEBRATES. 



Fig. 298. — reincola pluiladiformis. 



mented with radiating ribs, arranged mucli like those in Pholas, whence the specific 

 name. The shell cannot be completely closed, but gapes, while the mantle is almost 



entirely united in front, leaving but a slight open- 

 ing for the small, pointed foot. It has a rather 

 extensive distribution, reaching from Nova Scotia 

 to Texas ; and Professor Verrill writes that he has 

 received specimens from the Gulf of California which 

 are scarcely to be distinguished from this. It is 

 sometimes found among rocks and stones, below 

 low-water mark, but more frequently it makes deep burrows in stiff mud or clay 

 bottoms, climbing ujj and dowi\ in them by means of its slender and flexible foot. 

 The siphons are very long, and are united for only a short distance. Other species of 

 Petricola excavate and occupy burrows in the softer limestone rocks, and from this fact 

 the generic name has been given. I am not aware that this habit has been noticed in 

 our species. The shell of P. pholadiformis reaches a length of two and a half inches, 

 but specimens over t'ftro inches long are uncommon. In color it is a chalky white. 



The Macteid^ embraces a considerable number of trigonal equivalve shells, which 

 can be completely closed, or which gape slightly. The ligament is usually internal, and 

 contained in a deep pit, but occasionally it is external; the hinge has two diverging 

 cardinal teeth, and laterals are 

 frequently present. The out- 

 side of the shell is covered with 

 a thick epidermis. The pallial 

 sinus is short, and rounded or 

 angular, and the siphonal tubes 

 are united and have their ex- 

 tremities fringed with small 

 tentacles. 



Mactra soKdissima and the 

 closely allied Jif. ovalig are 

 known along our northern 

 coasts as hen-clam, sea^clam, 

 and surf-clam, these names 

 properly belonging to the more 

 common and more littoral spe- 

 cies first mentioned. The first is distributed from the Carolinas to Labrador, while 

 the second is only found north of Cape Cod. On every sandy beach JIf. soKdissima 

 occurs in large numbers, and after gales the other species is thrown up on the shore. 

 They are used in a limited way for food, and when properly prepared they make a 

 chowder no less palatable than the Mya arenaria, so extensively used. The toughness 

 of the older individuals prevents their more extensive use, and their preparation calls 

 for the use of the chopping-knife. The species are very active, and, instead of leading 

 sedentary lives in burrows, they plough through the sand by means of their well-de- 

 veloped foot. They have a considerable leaping ability, and many a time have I 

 known a basket full at night, and left in a 'trap,' to be half emptied in the morning, 

 the clams leaping out by means of their foot. The species of Mactra are valuable for 

 the student of molluscan anatomy, from the fact that their ganglia and nerves are col- 

 ored a reddish hue, and are thus more easily distinguished, and dissected out, than in 



Fig. 299. — Mactra ovalis. 



