372 



ROBERT TRACY JACKSON ON THE 



zards Bay. The prodissoconch is oval, rounded and not sharply defined from the suc- 

 ceeding dissoconch, as both have nearly the same shape and the same peculiar violet 

 color. The prodissoconch is marked by a fine pitted structure which partially obscures 

 the lines of growth. The dissoconch bears closel}'' crowded lines of growth and is vio- 

 let or amethyst color over its whole surface. In the adult, on the other hand, the shell is 

 mostly white or rosy, the amethyst color ])eing limited to the umljonal and posterior por- 

 tion of the shell. Gould says this species is commonly taken for the fry of Venus mer- 

 cenaria on account of the color of the tip (young). This is an unnecessai-y mistake as 

 the lunule, elevated ridges and white color of young individuals of that species (PL xxx, 

 figs. 9-10) are wanting both in the young and adult of Tottenia. 



Scrohlcularia tenuis, PI. xxx, fig. 11, illusti-ates a specimen from the collections of 

 the Smithsonian Institution. It has a distinct prodissoconch which is not, however, as 

 sharply defined as in many genera because the succeeding dissoconch is so like it in foi-m 

 that no marked contrast is produced by the inti-od notion of that latei- stage. The pro- 

 dissoconch shows concentric lines of growth and its limits are defined by a varix of 

 growth indicating the close of the stage. 



Saxicava arctica, Desh. Young specimens were collected on the roots of Laminaria 

 at Cohasset and Chelsea Beach in September, 1889. A very sharply defined prodisso- 

 conch exists at the umbos, fig. 50. It is ovate with uml)OS directed anteriorly and shows 



fine concentric lines of groAvth. The 

 succeeding dissoconch is oblong, cov- 

 ei-ed with a delicate epidermis, as is the 

 prodissoconch and bears comparatively 

 coarse strongly defined lines of growth. 

 The ligament is external and forms a 

 prominent feature of this early stage as 

 well as in the adult. Two ridges which 

 extend posteriorly from the limits of the 

 prodissoconch valves are ornamented 

 with spines. The spines vary in size 

 and number, but persist in s])ecimens up to four or five millimeters in length after which 

 they were not observed. Their existence is noted in the young of Saxicava in Binney's 

 Gould. 



3Iya arenaria, L. A number of young living individuals of this species were found 

 actively crawling on the glass slides in the drain-pipe traps at Buzzards Bay during 

 August, 1888; the spec imens varied from 0.6 mm. to 5 mm. in length. At Chelsea Beach, 

 Mass., in September, 1889, many clams wei'c also collected, from very young to those hav- 

 ing the adult characteristics fully developed. The specimens varied from 1.5 mm. in 

 length upwards, and instead of being free crawling, as were those collected at Buzzards 

 Bay, they were for the most part burrowing in a bed of dead eel grass where the roots 

 and mud formed a comparatively solid mass. The burial was superficial and so abundant 

 were the little clams that a handful of the material might contain fifty specimens more or 

 less. 



In ver\' young Mya, PI. xxx, fig. 13, a remarkable condition of the siphon tubes exists. 



Fig. 50. — Youiii; Saxicava arctica, showing oval roiinded pro- 

 dissoconch and succeeding spinous dissoconch. The ligament is 

 external and prominent. X 38 diam. (Drawn by .J. H. Emerton.) 



