_ 7 — 



as the oyster or scallop, in wliieli only one adductor, the posterior, is 

 left (see figure 2, page 8). A line connects the lower sides of these 

 scars when there are two and since here the mantle or pallium was 

 attached, it is called the pallial line. In forms wdth. siphons this line 

 is folded near the posterior end, forming a notch which varies in 

 depth with the size of the siplion and is called the pallial sinus. 

 These sinuses, corresponding to the position of the siphons, are found 

 at the posterior or hinder end ; to distingnisli the right and left valves, 

 therefore, it is only necessary to hold the shell with tlie hinge up and 

 the siphonate end toward one, when the right valve will correspond to 

 t'he right hand and the left to the left hand. 



The outer surface of each valve show^s a prominence, near the point 

 where the two shells are joined, Imown as the umbo (see figure 3). 

 Around this are found more or less distinct concentric lines, called 

 growth lines, marking what were once the margins of the shell. The 

 valves are usually covered with a horny layer or pcriostracum which in 

 old or wave worn shells may largely or wholly disappear. The valves 

 are flexibly united and opened by a tough rubbery ligament. This is 

 really composed of two parts, one of which is always outside, while the 

 second is either folded within this, when the ligament is said to be 

 external, or may lie between the edges of the shell, when it is sometimes 

 called a cartilage and said to be internal. In any case the ligament as 

 a whole works against the muscles and tends to open the shell ; for this 

 reason the shells of dead clams gape. The ligament is usually posterior 

 to the umbo and therefore nearer the siphonate end ; on the same side 

 there is in some shells a depression called the escutcheon (see figure 11, 

 page 12), and in some a similar area called lunule in front of the 

 umbo. 



Often tiny soft-shelled crabs are found within the mantle cavity of 

 clams or other bivalves; these are commonly knoMTi as "oyster crabs" 

 from the fact that they are found in the oyster, or "pea crabs" from 

 their minute size and rounded shape. They are not crabs that the 

 clams have eaten (the food of all clams is microscopic) but are adult 

 and fully formed crabs (as is shown by the presence of eggs on many 

 specimens) that find shelter within the clam and perhaps filch their 

 food from the clam's supply. Such a relationship of "host" and 

 "guest' (here, to be sure, an uninvited "guest") is found in other 

 cases and is known as " commensalism " in distinction from "parasi- 

 tism" where the "host" is harmed. The presence of these crabs in 

 no way injures the clams for food — in fact, the oyster crabs are esteemed 

 by some a great delicacy, though it is seldom that enough may be 

 obtained to furnish more than a taste. 



The usage followed in regard to common names deserves a word of 

 explanation. There are but a handful of common names in the 

 English language for shellfish^ — clam, mussel, cockle, scallop, oyster — 

 and hence particular species have to be distinguished as "hard clams" 

 and "soft clams," "little-necked clams" and the like. The use of 

 these names is often so local that they have no significance ; what is a 

 "hard shell" in one part of the state becomes a "paper shell" when 

 contrasted with other species in a different part of the state. 



