REPORT ON THE GASTEROPODA. 117 



I do not know that it can fairly be separated from Trochotoma, Deslongchamps, 1 a Laissic and 

 Jurassic fossil. That genus is indeed said to be nacreous, and the received distinction between Plcuro- 

 tomaria and Trochotoma on the one hand, and Scissurclla and Schismope on the other, is that the 

 former are nacreous, which the two latter are not (see Tate, Appendix to Woodward's Manual, p. 38) ; 

 but Dr Gwyn Jeffreys has broken down one-half of that distinction by showing (Brit. Conch., vol. iii. 

 p. 282) that " the nacreous inner layer of Schismope crispata is very evident when the shell is broken 

 or has been accidentally exfoliated." He does not, it is true, say this expressly of Schismope, but he 

 classes it in the Family Scissurellidas, one of the characters of which he states to be that the shells of 

 that genus are " nacreous " (loc. cit.) I have not been able to establish the existence of this feature 

 positively in Schismope, but the failure is very likely due to the thinness of the shells and the haste 

 of a search to which much time could not be given. 



Being thus in doubt, I have accepted the genus on the principle that it has as much right to 

 exist as Scissurclla has, and that while that genus may be distinguished from Plcurotoniaria by size 

 and colour, Schismope may be separated from Trochotoma by the same characters, though, as regards 

 this last genus, the question of colour is somewhat more doubtful. 



At one stage of its development the shell of Schismope resembles a young TrocJms ; somewhat later 

 it is often indistinguishable from Scissurclla except by the peculiar form of its umbilicus as shown in 

 some of its species. In neither genus is the cut in the outer lip to be found in the young shell. 

 Only after several whorls have been produced is the fissure formed ; and the formation is effected, not by 

 the absorption of shell already formed, but by the leaving of a cut in the fresh shell as it grows, the 

 upper end of this rift being plugged up by degrees, so as to prevent its becoming unduly long. 

 In Scissurclla this process goes on so long as the shell growth continues. In Schismopie, on the 

 contrary, as the shell approaches maturity the front of the fissure is bridged across by a layer of shell, 

 the lip edge thus becoming continuous, and the rift being transformed into a foramen. It is thus 

 in the complete stage of the shell alone that the form of the lip-cut will serve to distinguish the two 



Species. 



1. Schismope tabulata, n. sp. 2. Schismope lacuniformis, n. sp. 



3. Schismope carinata, n. sp. 



1. Schismope tabulata, n. sp. (PI. VIII. fig. 7). 

 Station 24. March 25, 1873. Lat. 18° 38' 30" N., long. 65° 5' 30" W. Off Culebra 

 Island, West Indies. 390 fathoms. Pteropod ooze. 



Shell — Obliquely discoidal, finely ribbed and spiralled, flattened above, witb a very small 

 slightly depressed apex, impressed suture, very large round mouth, large lacuniform 

 umbilicus, and a last whorl carinated in its latter half by the prominent upstanding canal- 



1 This genus was published in 1841, and not, as is often asserted, in 1843. It is distinguishable from 

 Ditremaria, D'Orb. (1843) = Rimiilus, D'Orb. (1839), not Rimula,~DeiT., by having only one small oval foramen 

 instead of two united by an open canal. 



