183 



PLEUROTOMARIA, Defrance. 

 Plates I. II. 



Thes. pi. 491. 



Testa spiralis trochiformis, intus nacrea ; anfractu ultimo 

 a margine aperturse retro acute inciso ; anfractibus superio- 

 ribus, incisurse cicatrice spiraliter notatis. — Operculum 

 corneum. 



The shells of this remarkable genus have all the characters 

 of Trochus, being spiral and conical with pearly interiors. 

 They have at the same time the very important exceptional 

 character of an incision from the edge of the aperture back- 

 wards ; the cicatrix of the incision or slit appears in tlie 

 upper whorls, and continues spirally downwards to the back- 

 ward margin of the slit in the last whorl. In the growth of 

 the shell this slit has been filled up in all the whorls succes- 

 sively to within the required distance from the aperture. 

 The successively filled up holes of the Haliotis are believed to 

 be analogous to this. Also some of the Emarginulue exhibit 

 similarly filled-up openings. The microscopic iScissurella isa 

 still closer example, but it does not belong to the pearly tribe. 



Species. 



1. QuOYANA, f. 5, 6, Fischer. — Subdepressa conica, pallide 

 fulva, flammulis pallide rubris, oblique concentricis ornata, 

 liris spiralibus et striis concentricis elevatis oblique decus- 

 sata; anfractibus decern paulo planatis, ad suturam tumi- 

 diusculis; apicalibus subattenuatim acuminatis ; idtimo 

 versus marginem basalem oblique angulato ; disco basali 

 subplanato Isevi subumbilicato ; fissurse cicatrice planata ; 

 apertura oblique quadrilateral!, fissura inframediana, angiista 



