307 



THE MARINE MOLLUSCA OF SAO THOME, II. 

 Descriptions of a New Genus and Five New Species. 



By J. R. i.E B. TOMLIN, M.A., ano L. J- SHACKLEFORD. 



(Plate V.). 



(Re;id before the Society, Feb. 6th, 1915). 



^Tropidorissoia gen. nov. 

 This name is proposed for a group of Rissoids which seems 

 confined to the islands off tlie West Coast of Africa, the type 

 species selected being T. taphrodes, Tomlin & Shackleford, de- 

 scribed below. 



They are small conical shells of four to five whorls, with a varying 

 number of more or less prominent spiral keels which continue 

 almost or quite to the apex {cf. "Smith on Riss. platia), the inter- 

 spaces being smooth, and with much rounded apertures. 



We should assign to Tropidorissoia the following S. Helena 

 species described by Mr. E. A. Smith, viz. Rissoa aegua, R. platia, 

 R. wallichi, R. perfeda, R. varicifera, and probably R. vaga and 

 R. siniulam. 



Tropidorissoia ^taphrodes sp. nov. (pi. 5, f. 5). 



Shell minute, conical, strongly carinate, slightly rimate or per- 

 forate, smooth, solid, light brown in colour, somewhat shining ; 

 whorls 5, protoconch smooth and of the same colour as the rest of the 

 shell ; the last whorl has three keels, one in the middle of the whorl, 

 and, of the other two, one above and one below at about equal 

 distances from the central keel, which is rather more prominent 

 than the upper and lower ones ; the keels are very well marked, 

 massive and completely rounded ; the other whorls each have two 

 keels ; the last whorl occupies about half the shell ; suture indistinct ; 

 aperture almost round : peristome acute ; columella slightly arched, 

 reflexed. 



Long. : I '5 mm. ; diam. max, i mm. 



Hab. : S. Thome, not very common. 



The type specimen is the only one that we received in a live 

 state. 



This curious little species, which we make the type of a new 

 genus, is only comparable with certain S. Helena species already 

 mentioned. 



1 Tp07rtS=.^ keel. 



2 P.Z.S., 1890, p. 309. 



3 ibid. pp. 288-290. 



4 Tac^OwS^JS^^treneh-like, 



