416 
THE GE0L03IST. 
and then diverging to form the dental plates in a very similar manner to 
what we find in Pentamerus. The fissure is covered by an arch- 
shaped deltidium; but in C Demai-lii, Mr. Bouchard has remarked 
that the median septum is continued as far as under the surface of the 
deltidium, so that the dental plates are fixed to its sides instead of to the 
upper edge, as in Cyrtina heterodyta and C. septosa. The arrangements 
in the smaller or more important valve are still unknown, notwith- 
standing the many efforts I have made to pry into the interior ; and 
since no vestiges of spiral coils have hitherto been noticed, the 
character of the genus or sub-genus cannot bo at present definitely 
determined ; it is to be hoped that those who reside in districts where 
some of the species of Cyrtina occur, will endeavour to discover the 
interior arrangements of the smaller or dorsal valve. 
The next sub-genus which claims our attention is the one named 
Sucssia by Mr. E. Deslongchamp ; but, as its interior characters are as 
yet imperfectly known, more information will be required in order to 
establish its generic or sub-generic claims. The two branches which 
constitute the first spiral coil are united by a transversal shelly band, 
from the centre of which proceeds another short lamella, which is 
directed towards the bottom of the valve. The species that compose 
this small group possess an unusually large hinge-plate, as well as two 
singularly-shaped appendages, which, arising from the inner socket 
walls, follow an inward direction. No other member of the Sj)irifcrtdce 
has presented a similar feature. 
(To be conliiiucd.) 
ON ROCKS ; THEIR CHEMICAL AND MINERAL COMPOSITION, 
AND PHYSICAL CHARACTERISTICS.* 
By H. S,, Plymouth. 
1, — General Introduction. 
I. " II suffit de parcourir des parties tres circonstrites de la surface du 
globe, pour voir qu'ellc est composce de materiaux tre3-varies."f The 
* In case any of my readei-s, bning young geologists, should misapprehend the 
exact geological meaning of the word rock, I give Sir Charles Lyell's definition: — 
" The materials of the earth's crust are not thrown together confusedly ; but 
distinct mineral masses called ' rocks ' arc found to occupy definite spaces. The 
term rock is applied indifferently by geologists to all these substances, whether they 
bo soft or stony ; for clay and sand are included in the term, and some have even 
brought peat uuderthis denomination." — "Elements," p. 2. 
I D'Aubuissoa's " Geognosie" par Burat. 
