OF MASSACHUSETTS. 37 



byssus, formed by a gland in the foot (Fig. 30). It is interesting to 

 note that in each case the attachment, though entirely different, comes 

 at the end of the prodissoconch period, and that the organs of attach- 

 ment and locomotion, owing to the absence of the foot in the oyster, 

 are strikingly dissimilar. 



The Shell The shell of the dissoconch stage (Fig. 19) is sharply 

 separated from the prodissoconch by a well-defined growth line and by 

 different shell formation. The prodissoconch has a smooth homogeneous 

 structure lined with finely concentric lines of growth. The new growth 

 is of an entirely different character, as the right or lower valve acquires 

 a prismatic structure (Fig. 41), such as was described in Ostrea and 

 Pecten by Jackson (4), in which each prism is separated by an inter- 

 vening space. The structure on the left or upper valve, while not pris- 

 matic, is readily discernible in appearance from the prodissoconch. 

 The first indications of coloring in the shell appear during the latter 

 part of this period as little dashes of yellow or brown. The dissoconch 

 shell has a smooth, even appearance, with no plications, and separated 

 by regular concentric growth lines, which are used by the writer to 

 mark off certain sub-stages. Probably these growth lines, as yet not 

 eroded by action of water or subsequent growth, denote daily periods, 

 tides or other intervals in shell formation. 



With this stage the hinge becomes for the second time a straight line. 

 During the first three sub-stages it is narrow, hardly four-sevenths the 

 width of the animal, but later it increases relatively in width, until 

 at the beginning of the plicated stage it is nearly the same length. In 

 the early stages the hinge line is not absolutely straight, but inclines 

 slightly upward at both ends. The inside of the hinge line is set with 

 teeth, as described for the prodissoconch veliger (Fig. 22). 



The form of the scallop gradually changes during the dissoconch 

 period, as it grows from .18 to 1.20 millimeters. At first the shell 

 rounds out anteriorly, while posteriorly it breaks directly down from 

 the hinge line with a slight curve (Figs. 19, 20). The left or upper 

 valve elongates anteriorly a slight distance beyond the right, covering 

 in this region the byssal notch of the lower valve (Fig. 19). At a 

 slightly later stage (Figs. 25, 26) the shell has formed in this region 

 a " pseudo ear," which disappears as the animal grows larger, and 

 again reappears at the size of 1.50 to 2 millimeters to form the true 

 "ears" on both sides of the shell. Meanwhile, the posterior part of 

 the shell has increased slightly faster than the anterior, causing the 

 prodissoconch to assume a position anterior to the center (Fig. 28). 

 Toward the close of the dissoconch stage the scallop loses its elongated 

 form and takes on a semicircular appearance (Fig. 31). The various 

 changes in form from the early veliger to the 2 millimeter scallop are 

 shown in Fig. 77, which consists of eleven concentric camera outlines 

 of different sized scallops. 



