256 M. M. D'Orbigny & G. B. Sowerby on Scissurella. 



the Scissurella fills up among the Trochoidce, a chasm in the series 

 of conformations, observed in the aperture of the Shells of different 

 families of the Mollusca and Annelidce. The slit, which is the 

 principal character, and from which the genus is named, positively 

 indicates a peculiar organization in the animal which inhabits it, 

 and has appeared to me of sufficient importance to authorize the 

 formation of a new genus ; the following are its characters. 



Scissurella. Testa univalvis, libera, umbilicata, spira depressa, 

 apertura, subrotundata, labiis disjunctis, canali peristomateque nul- 

 lis : margine dextro labii superne profunde emarginato ; emargina- 

 tione incrementum anfractuum secuta, fere usque ad marginem 

 aperturae obliterate, et carinam in dorsum testae notante. 



Shell univalve, free, umbilicated, with a depressed spire, aper- 

 ture rather rounded, without a canal, its lips without a peristome 

 and disjoined : right lip notched with a deep slit, following the 

 growth of the volutions, obliterated to within a short distance of 

 the margin, and forming a sort of keel upon the back of the 

 shell* 



Species 1. 

 Scissurella Iwvigala, testa, ovali, laevi, alba, translucente, tenui, 

 fragili, apice depresso, anfractibus tribus, lineis incrementi tantum 

 notatis. 



shall not be justified in pronouncing such a judgment until an actual acquain- 

 tance with the appearance of those genera as they emerge from the egg, shall 

 enable us to do so with precision, yet we need not hesitate to suggest the pos- 

 sibility, of the Scissurella proving to be either an Haliotis or a Fissurella just 

 emerged from the egg ? 



* Supposing from the fact I have before stated, that the slit or notch men- 

 j tioned in this character only retains that form while the shell is quite young, 

 V and that asit arrives at its more mature age the margin becomes entire, I should 

 J propose the following as an alteration of the character of the genus; " prope 

 \ marginem dextram foramine oblongo, incrementum anfractuum secuto, et carinam 

 in dorsum testa notante." " An oblong foramen near the right lip, following the 

 growth of the volutions, and forming a sort of keel upon the back of the shell. I 

 think also that the words " perhtomate acuto" should be added to the charac- 

 ter of the genus, as it will be observed that the edge of the peristome is de- 

 scribed as sharp in every species. 



