372 



ROBERT TRACY JACKSON ON THE 



zards Bay. The prodissoconcli 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 closely 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 being limited to the lunbonal 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 unnecessaiy mistake as 

 the Imiule, elevated ridges and white color of young individuals of that species (PI. xxx, 

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



Scrobiciilaria tenuis, PL xxx, fig. 11, illustrates 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 form 

 that no marked contrast is produced by the introduction of that later stage. The pi'o- 

 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 lunl^os directed anteriorly and shows 



fine concentric lines of growth. The 

 succeeding dissoconch is oblong, cov- 

 ered with a delicate epidermis, as is the 

 prodissoconch and bears com|iaratively 

 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 Avhicli 

 extend posteriori}^ from the limits of the 

 prodissoconch valves are ornamented 

 with spines. The spines vary in size 

 and number, but persist in specimens up to four or five millimeters in length after which 

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

 Gould. 



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

 actively crawling on the glass slides in the di-ain-pipe traps at Buzzards Bay dui'ing 

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

 Mass., in Sejitember, 1889, many clams were 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 gi-ass where the roots 

 and mud formed a comparatively solid mass. The biu'ial was superficial and so abundant 

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

 less. 



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



Fig. 50. — Young Saxicava arctica, sliowiiig oval roiiiided pro- 

 dissoconch and succeeding spinous dissoconch. Tlie ligament is 

 external and prominent. X 38 diam. (Drawn by J. II. Enierton.) 



