On Gemma, Parastarte, and Psephidia. 473 



54. Chonophorus transandeanus, G until. 

 R. Condoto (Spurrelf). 



Symbranchidae. 

 55. Symbranchus marmoratus, Bloch. 

 R. Condoto (Spurrell). 



LVII. — On the She/Is knoion as Gemma, Parastarte, and 

 Psephidia. By A. J. Jukes-Beowne, B.A., F.R.S. 



These small shells are aberrant members of the VeneridaB, 

 which have sometimes been regarded as independent genera 

 and sometimes as subgenera of Venus or Chione. A recenl 

 examination of specimens under the microscope has disclosed 

 the fact that several mistakes have been made with regard 

 to their internal characters, and has also revealed some 

 features which seem to have escaped notice. I propose, 

 therefore, to give a more complete and accurate account ol 

 these shells, and to indicate a speeies which has been referred 

 to Psephidia (=Psephis), but which differs in so many 

 particulars that it must be regarded as a distinct subgenus, 

 and, as such, must receive a new name. 



1. Gemma, Deshayes. 



This little shell was first described as Venus gemma by 

 J. S. Totten in 1834*, and was for a long time supposed to 

 have only two teeth in the right valve, all the text-books 

 copying Deshayes's erroneous statement to that effect, 

 although it is quite clear that Totten and other American 

 conchologists knew quite well that there were three teeth in 

 each valve. 



The three teeth of the right valve are fairly well shown in 

 Totten's figure, and he described the dentition as follows: — 

 "Teeth divergent, the medial tooth of each valve stout and 

 triangular, the anterior tooth of the right and the posterior of 

 the left valve thin and not easily recognized." The shell 

 was accepted as a Venus by Gould in 1841 f, who described 



* ' American Journal of Science,' vol. xxvi. p. 366 (New York, 1834). 

 t Report mi Invert, of Massachusetts, p. B8, li-r. 51. 



Ann. (& M<i<j. N. Hist. Ser. 8. Vol. xii. ! 3 1 



