320 Mr. J. G. Jeffreys on the genus Scissurella. 



by no means satisfied with the general opinion that it belongs to 

 the TrocMdce ; although I believe it is allied to that family in 

 respect of both the shell and animal. Nearly half a century ago, 

 Colonel Montagu supposed the S. crispata of British authors to 

 be the fry of a Trochus ; but since his time many other species 

 have been discovered in various parts of the world, all of which 

 exhibit the peculiar structure of Scissurella; and the recent de- 

 scription by Mr. Barrett of the external organs, as well as my 

 own observation of the operculum in another (although pro- 

 bably not a congeneric) species, have afforded additional data for 

 ascertaining the true relations of the genus. With respect to 

 the presence or absence of an operculum, I may remark that 

 even in the same genus {Mangelia) some of the species are oper- 

 culated, while others are inoperculate. Ciliated appendages, 

 simple filaments or cirrhi* varying in disposition and number, 

 and combined with opercula of different forms (which however 

 are sometimes wanting), occur in Macgillivrayia and Ckeletropis 

 (pelagic mollusks), as well as in Lacuna, the Fissurellada and 

 Patella, which are widely separated from the TrochidcB and from 

 each other. I submit that the question ought not to be deter- 

 mined upon analogical considerations alone. The orifice in the 

 mantle and shell of Scissurella indicates an affinity to Emarginula 

 and Fissurella, the young of which are well known to be spiral. 

 The slit in S. striatula, Ph., does not commence until the animal 

 is half-grown. Its sides or walls ai'e raised above the surface of 

 the shell, and present a prominent ridge ; a hollow groove being 

 thus apparently formed for the reception and passage of the ex- 

 current or anal canal. The foramen in which it terminates is 

 oblongo-fusiforra, being usually more pointed in front; and it 

 projects like the groove, in this respect resembling an analogous 

 process in the young of Fissurella. As Mr. Alder justly re- 

 marks, this conformation appears to exhibit the same relation be- 

 tween this species and Scissurella as Puncturetla bears to Ema?'- 

 ginula. The ribs generally cease when the slit begins to be 

 developed ; and it would seem as if that operation altered or in- 

 terfered with the original secreting power of the mantle, which 

 afterwards was applied as well to the filling-up of that part of 

 the slit which became useless as to the construction of a series 

 of close-set transverse strise or steps between the sides of the 

 groove. The foraminal termination of the slit I have only ob- 

 served in this species (S. striatula), although a great number of 

 the British species (>S. decussata or crispata) have been examined 



* Obs. The use of these organs seems to be Uttle known. In Macgil- 

 livrayia they are supposed to serve for prehension as well as natation ; but 

 in the majority of eases they probably perform the functions of supple- 

 mental tentacula. 



