202 SHELLS AND SHELL-FISH. PART I. 



genus deserves attention ; they are so far imbricate 

 that one overlaps the other to full one half of their 

 length, so that the suture of the basal whorl extends 

 half way up that which precedes it, and gives the ap- 

 pearance of the outer lip ascending on the spire. But 

 among the fossils figured by De Ferussac, there are some 

 which have an aspect altogether different: the spire, 

 instead of being acutely pointed, is obtuse, and so short 

 as to consist of only three whorls ; the aperture is also 

 represented as much shorter than in any of the recent 

 species, and the base more effuse. Again, there are two 

 species*, which perfectly resemble the sub-genera Me- 

 lacantha and Hemimitra, being short, broad, and coro- 

 nated with a single row of short spines, the tip of the 

 spire being acute. Now, as these two types obviously 

 accord with Potadoma and Melacantha in the adjoin- 

 ing circle of the Melanice, we have not scrupled to 

 designate them as sub-genera, distinguishing the first 

 as Melanithes, as being yet only known in a fossil state, 

 and the second as Canthidomus, from its little spines. 

 The confidence we repose in the accuracy of Ferussac's 

 beautiful but costly plates, induces us to do this without 

 having ourselves seen any of these shells : but the cha- 

 racters we have stated cannot be doubted ; and the links 

 by which these two presumed types are connected with 

 the recent sub-genus Melanopsis are so perfect, that they 

 have every indication of being natural sub-genera. t We 

 must now notice a fifth type, which unites something 

 of the characters of those two very opposite sub-genera, 

 Canthidomus and Melafasus; this is our sub-genus 

 Melatoma, founded upon a remarkable Ohio shell sent us 

 many years ago by our old friend professor Rafinesque. 

 It has the general form of a Pleurotoma and of Mela- 

 fusus, with a well-defined sinus or cleft near the top of 

 the outer lip ; while the inner, though thin, is somewhat 



* Plate Melanopsidce, fig. 16. 7. Plate 2. figs. 9, 10. 



t Thus the species at pi. 2. fig. 12. plainly connects Melanithes with Can- 

 thidomus ; while those on pi. 1. fig. 14, 15. seem to be aberrant to the last 

 sub-genus, leading to the long-spired Cerithidlce. Fig. 6. pi. 2. is clearly 

 a Pirena, and fig. 8. a Cerithium. 



