370 ROBERT TRACY JACKSON ON THE 



conch, p, exists and is succeeded by considerable dissoconcli growth. The prodissoconch 

 is gloliose and ahnost circidar, sliglit umbos being developed. "When young, fig. 4, it is 

 very translucent of a light horn color, liut in older specimens, fig. 5, the shell is thick- 

 ened by shell deposition, and it is darker and more opaque. The prodissoconch in Sphae- 

 I'ium is of gigantic size compared with the prodissoconch found in other Pelecypods de- 

 scribed and figured in this paper. PI. xxx, fig. 5a, is a yonng specimen magnified only 

 ten times in order to show the large size of this early shell. By definition, the jirodisso- 

 conch in Pelecypods is the shell of the completed embryonic period, and it is natural that 

 in Sphjei'ium we should have a large jirodissoconch because of the relative great size of 

 the embiyo in that genus. 



Besides in Splicerium securis, a similar strongly marked prodissoconch has been ob- 

 served in 8. partumeium, Say, S. rhomhoideum, Adams, and S. transversum. The 

 specimens of these species observed are in the choice Beecher collection of fresh water 

 shells, in the New York State Museum. 



Echinocliama (Chama) arcineUa, L. In a specimen sent me by Dr. Beecher, a pro- 

 dissoconch was found intact. It is ovally rounded with umbos directed anterioi-ly. The 

 nepionic stage is pink in color, is ovately drawn out, and truncated posteriorly. Con- 

 centric ridges mark the nepionic period and as pointed out by Fischer, this early stage 

 resembles a Venerupis in form. The shell of the nepionic stage is equivalvular as ob- 

 served in several specimens in the collections of the Boston Society of Natural History. 

 As soon, however, as the right valve becomes attached to a foreign body (which occurs 

 when the shell is about 3 mm. in length), both valves immediately change their form and 

 assume the irregular growth characteristic of the Chamas, and soon the spines and jiit- 

 like depressions characteristic of the species are introduced. We find in this case a full 

 substantiation of our views on the modifying effects of the conditions of fixation on 

 shell form as fully discussed in section vin, p. 322. 



Petricola plioladiformis. Lam. At Nantucket in July, 1889, a large number of young 

 specimens of this species were collected, varying from very small to 12 mm. in length. 

 The largest individuals had the adult charactei's fully developed. The specimens were 

 burrowing in shallow Mytilus beds which covered rocks between tide marks. In the 

 youngest specimens found, PI. xxx, fig. 6, also in some older individuals, a distinct pro- 

 dissoconch, p, exists at the umbos. It is not widely different from the succeeding disso- 

 conch in form, but is sub-ovately rounded and appears like a cap on account of its 

 whiteness compared with the succeeding grayer dissoconch. The whiteness of the pro- 

 dissoconch is noted in several genera in this section, and as described in Mytilus it is 

 doubtless due to the wearing away of the organic epidermis of the valves and consequent 

 exposure of the chalky-white lime basis. The early dissoconch of Petricola, PI. xxx, fig. 

 6, is strikingly different from the adult. It is oblong, anteriorly roundly compressed and 

 l)osteriorly slightly truncated. The lines of growth indicate an earlier more .oval form. 

 In this eai'ly stage the ligament, Z, projects from the hinge line as in the later stage, PI. 

 xxx, fig. 7, and in the adult; but no plications of the shell exist until considerably later. 

 The specimens, although taken from their bui-rows in the Mytilus bed, proved to be 

 active crawlers having a highly developed foot, fig. 6. The siphons of this early stage 

 fig. 4, are separate for the entire length produced beyond the limits of the shell. The 



