DE. GWTN JEFFREYS OX THE GENUS SEGUENZTA. 



605 



umbilicus, which it largely conceals, with a deep narrow furrow 

 behind it. It advances thin and pointed, curving over to the 

 right to its angular junction with the basal lip. Umbilicus de- 

 fined by a spiral thread and with two other spirals within it. 

 It is not so much small as concealed by the pillar-lip. H. 0-82. 

 B. 0-63, least 0-53. Penultimate whorl 0-19. Mouth 0-4 ; 

 breadth 0-38. 



Perhaps Trochus {Cantharidus) iris, Humph., while totally 

 unlike in most respects, nevertheless approaches this in form 

 more than any other shell does. 



Note as to the position of the Genus Segiienzia among the Gas- 

 tropoda. By J. GwTN Jeffreys, LL.D., F.R.S , F.L.S. 



[Read February 6, 1879.] 



The Kev. E. Boog Watson in a valuable paper, which was lately 

 read before the Society, on some of the Mollusca procured by the 

 * Challenger ' Expedition, included the genus Seguenzia in the 

 Trochus family. 



"When I described that genus in the ' Proceedings of the E-oyal 

 Society ' (No. 73; 1876, p. 200), I said that it evidently belonged 

 to the Solarium family ; and I gave my reason for this opinion, 

 to which I adhere. I am sorry not to agree with my friend Mr. 

 "Watson ; but such difi'erence of opinion is useful to science. 



The chief points of distinction between the genera of TrochidaB 

 and Solariidse consist in the shell of the latter not being pearly 

 or nacreous inside, and in the operculum being ear-shaped and 

 few-whorled, with a lateral nucleus and excentric spire, as in the 

 Littorinidse. The operculum in the TrochidaD is circular and mul- 

 tispiral, with a central nucleus. I have fortunately succeeded in 

 extracting the operculum from a small fresh specimen of Seguen- 

 zia formosa ; and I find it to be ear-shaped, very thin, paucispiral 

 (having two whorls only), the spire being very small, excentric, 

 and placed on the columellar side. It resembles that of Solarium 

 and Adeorbis, genera of the same family. Mr. "Watson says that 

 my account of the operculum represents " a feature, which, accord- 

 ing to Quoy and Gaimard is shared by Euclielus, Philippi's sub- 

 genus of Trochus y Now all that Philippi remarks as to the oper- 

 culum of Euchelus is, " der Deckel hat nach Quoy und Gaim. nur 

 wenige "Windungen ;" but no mention is made of the shape of the 

 operculum nor of its spire and nucleus. I have carefully examined 



LINN. JOURy. — ZOOLOGY, VOL. XTY. 45 



