238 



ON THE GENUS GEMMA, DESHAYES. 



By WILLIAM HEALEY DALL. 



(Read before the Society, May 14, 1902). 



In 1834^ Capt. J. G. Totten described under the name of Ve?ius 

 gemma the pretty Uttle shell which is the subject of these remarks. 

 His description is as follows: — "Shell subrotund, nearly equilateral, 

 concentrically furrowed, glossy, anterior portion and basal margin, 

 both within and without, white or pale reddish violet, remainder red- 

 dish purple, darker at and near the superior and posterior margins; 

 no lunule; beaks small, incurved, separate, generally eroded; teeth 

 divergent, the medial tooth of each valve stout, triangular, the anterior 

 tooth of the right and the posterior of the left valve, thin and not 

 easily distinguished; inner margin crenulate. Length o"i5 of an 

 inch. The length being represented by 13, the breadth [height] will 

 be 12, and the diameter 6." 



"Inhabits the coast of Massachusetts and Rhode Island." 



"I found this beautiful little shell first on the beach at Provincetown, 

 Cape Cod (Mass.); it has since been found in Newport Harbour. The 

 largest specimen I have seen among many hundreds is barely ^^ of an 

 inch in length; the more common length being about ~ of an inch. 

 It is often much eroded on the disks and then the colour is bluish 

 white." 



Capt. Totten's figures are in a coarse woolly lithography, the 

 engraver does not indicate the crenulations of the margin or the lateral 

 teeth. The form figured is that from Massachusetts Bay. In this 

 description the cardinal teeth are correctly described, but the laterals, 

 and the faint boundary of the lunular area, were not noted. 



In 1842 H. C. Lea" describes Cyrena p^irpurea from Delaware Bay, 

 saying that he had supposed it to be the same as Totten's Venus ge/nma, 

 but on examination found that the hinge "is like that of Cyrena" and 

 the internal margin not crenulate. This indicates that he had cor- 

 rectly identified the teeth which are like those of Cyrena, except that 

 the laterals in Gemma are relatively weaker and the cardinals are 

 not bifid or grooved. The crenulation of the margin is present or 

 absent at different stages of growth in the same specimens. As a 

 matter of fact Lea's C. purpurea is a young specimen of the southern 

 sub-species of V. gemma. His types, from the collection of Dr. Isaac 

 Lea, are now in the National Museum. They are so small and 

 internally polished that the sinuated pallial line is not visible. 



1 Aii?er. Journ. Sci., vol. 26, p. 367, fig. 2, a, b, c, d, Nov., 1834. 



2 Ainer. Journ. Sci., vol. 42, p. 106, pi. i, fig. i. 



