OF MASSACHUSETTS. 13 



water from 5 to 30 feet deep. Scallops are often abundant on the high 

 flats where there remains but a foot or two of water at low tide, as on 

 the Common Flats at Chatham. These exposed places with the thick eel 

 grass seem to receive the heaviest sets, but the young often perish in 

 the cold winters. Scallops can arbitrarily be separated into two classes : 

 (1) the channel or deep-water scallops, found in water 15 to 60 feet 

 deep, and (2) the shallow-water or eel-grass variety, from low-water 

 mark to 15 feet. 



While the extent of the scalloping area is large, owing to the wide 

 range of the animal, only portions are ever productive at any one time. 

 A set may be in one place this year and the next year's spawn may 

 " catch " in a different locality. Thus, while all the ground is suitable 

 for scallops, only a small part is in productive operation each year. 

 The natural barrier to the distribution of the scallop is the exposed 

 nature of the coast, as this mollusk cannot live in rough waters. 



CHAPTER II. ANATOMY. 



The loss of the anterior adductor muscle in Pecteu, as shown by 

 Drew (1) and Jackson (4), has been accompanied by a shifting of the 

 soft parts, so that the antero-posterior axis, instead of running parallel 

 to the hinge line, forms with it an angle of at least 60. While this 

 fact is recognized, for simplicity the relation of the hard and soft parts 

 in the following description is considered as in a typical lainellibranch. 

 The animal is oriented: (1) dorso-ventral axis or height, from the 

 hinge to the opposite edge of the shell; (2) antero-posterior axis or 

 width, the distance across the shell; (3) lateral axis or thickness, the 

 distance between the two valves (Fig. 65). 



The Shell. The scallop shell consists of two lateral valves joined 

 together on the dorsal edge, the hinge line (HH), by means of a thin 

 ligament. The two valves, which open on the ventral or lower edge, 

 are nearly round and of equal curvature. The right or lower valve, on 

 which the animal rests, is of a lighter and cleaner color and differs from 

 the upper in having a byssal notch (B) or foot groove. In the scallop 

 less than a year old this notch is lined with several projecting teeth, 

 which are absent in the old animal. If the shell of an adult is broken 

 along the notch a row of thirty-five or more of these teeth can be seen 

 extending back to the urnbo. The adult scallop is somewhat wider than 

 high, the average dimensions being: height, 2% inches; width, 2% 

 inches; thickness, IM-O inches. 



On the outer surface of the shell are prominent ridges and furrows 

 (R, F) which radiate from the beak to the free margin, giving the 

 animal a beautiful fan-like appearance. The number of ridges does not 

 vary in the individual scallop, the adult having the same number as 

 the very young animal. In different scallops the number of ridges 



