OF CONCHOLOGY. 183 



The geographical distribution of these two genera of large 

 univalves, both in the recent and fossil state, is very interesting, 

 inasmuch as the recent forms belong exclusively to the coasts of 

 the West Indies, the Gulf of Mexico and the North Atlantic, 

 and the fossils to the tertiaries of the Atlantic slope. 



Whilst the distinction between spinous and tuberculated forms 

 is quite decided amongst the recent shells of the two genera, it 

 is less marked in the fossils, and all are alike, when young, in 

 having tubercles only. The difference is that in one genus the 

 tubercles remain in the adult, but in the other, in a certain 

 stage of growth, they become, though not invariably, open or 

 foliated spines. This fissure is in the direction of the shell's 

 growth, and is the only difference between the spines of B. 

 aruanum and the tubercles of S. coronatus. The spines some- 

 times appear to be a prolongation of certain prominent lines of 

 growth in the shell. This character seems to have suggested 

 the name aruanum applied to the typical species. 



Quite young shells of the canaliculate group, as &'. coronatus 

 and rugosus, are not canaliculate. 



There is no prominent fold on the columella of these genera, 

 but only a slight oblique groove. 



The first of this group of genera or subgenera appears to 

 have originated in the Eocene, but it is only in the Miocene 

 that it assumes its maximum development and marked diversity 

 of characters and forms. 



The species of Sycotypus, figured by Lister, t. 878, is un- 

 known to me. Dillwyn refers it to canaliculatus, but the adult, 

 young shell, embryo and ovaries are all very different from those 

 of canaliculatus. 



SYCOTYPUS, Browne, Gill, (not Gronovius). 



Recent species. JSIiocene species. 



S. CANALICULATUS, Lin. S. ALVEATUS, Conrad. 



^ S. PLAGOSUS, Conrad. S. coronatus, Conrad. 



var. clegans, Conrad. S. CAROLINENSIS, Conrad. 



-. S. PYKUM, Dillwyn. S. canaliferus, Conrad. 



FuJgur pyruloides, Say. ^. S. incilis, Conrad. 



*. S. EXCAVATUS, Conrad. 



Cassidulus carolinensiSy 

 Tuomey and Holmes. 

 S. RUGOSUS, Conrad. 



