242 JOURNAL OF CONCHOLOGY, VOI,. ID, NO. 8, OCTOBER, I902. 



is often the case in tottenii. The pallial sinus averages smaller and 

 more rounded apically. This form is also paler on the whole than 

 the tottenii^ and more frequently straw colour or entirely free from 

 purple, though sometimes quite dark colored. 



The following proportional (not actual) measurements in millimetres 

 indicate the differences: — 



G. 



Length 



Height 



Diameter 



Height of the line of \ 



greatest length above \ 145 12-0 



level of base } 



The figures above cited are from typical examples measured on 

 magnified outlines secured by a camera lucida. 



The strongly sulptured southern form of G. purpurea was referred to 

 Parastarte^ under the manuscript name of P. concentrica. I was led 

 into this error by the erroneous descriptions of the hinge of Gemma 

 given in the text books, and by the fact that, except that it has lost 

 the feeble cardinal tooth in each valve, the hinge in the Parastarte is 

 the same, and both are similarly colored and viviparous. 



The correct description of the hinge of Gemma purpurea, in good 

 order and fully developed, is as follows: — Right valve: anterior 

 cardinal margin with a small sharp ridge representing an anterior 

 lateral tooth, in the same radiating line from the beak as the anterior 

 cardinal, but the lower end of the anterior cardinal and the upper end 

 of the anterior lateral are separated by a distinct gap; the anterior 

 cardinal is thin, elongated and rat-her feeble; the middle cardinal is 

 strong and triangular; the posterior cardinal is lamellar and distinct; 

 the posterior margin is deeply grooved to received the posterior lateral 

 of the opposite valve. Left valve: — anterior margin grooved for the 

 lateral of the right valve; anterior and middle cardinal teeth stout and 

 triangular but not so short as the middle cardinal of the right valve; 

 posterior cardinal elongated, feeble; posterior margin raised into a 

 feeble ridge representing a posterior lateral. Siimmaij: each valve 

 has two strong and one faint cardinal, one lateral tooth, and one 

 groove for the opposite lateral. The anterior cardinal in the right 

 valve and the posterior one in the left are feeble. Formula ^^^l^ 

 The feeble teeth are often broken off, and sometimes undeveloped 

 in the young, or obsolete in eroded adults." A perfectly developed 

 and uninjured adult will always show the above described dentition. 



The siphons are wholly disunited, the anal is shorter, hyaline, with 

 a protruding valve when extended; the branchial is fringed at its 



I B%Ul. U.S. Nat. Mus. no. 37, p. 



