416 



tllE GEOL03IST. 



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; bat in C. Demarlii, llr. 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 Tieteroclyta and C. septosa. The arrangements 

 in the smaller or more important valve are still unknown, notwith- 

 standing the many efi'orts 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 be 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 

 Suessia 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. ITo other member of the Bj^iriferidct 

 has presented a similar feature. 



{To he continued.) 



OUT ROCKS ; THEIE CHEMICAL AND MINEEAL COMPOSITIOI^, 

 AND PHYSICAL CHARACTEEISTICS.-^^ 

 By H. S., Plymouth. 

 1 . — General Introduction. 

 1. " II suffit de parcourir dcs parties fres circonstrites de la surface du 

 globe, pour voir qu'elle est composce de materiaux tres-varies."! The 



* In case any of my readers, being young geologists, should misapprehend the 

 exact geological meaning of the Avord 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 ' are found to occupy definite spaces. The 

 term rock is applied indifferently by geologists to all these substances, whether they 

 be soft or stony ; for clay and sand are included in the term, and some have even 

 brought peat under this denomination."— "Elements," p, 2. 



t D'Aubuissou's " Geognosic" par Burat. 



