206 JOURNAL OF THE PLYMOUTH INSTITUTION. 
in its eastern portions (Cattedown, Oreston, &c.) they are compara- 
tively rare, and over considerable areas altogether absent. And in 
like manner the branching corals are found chiefly in the western 
area. There does not appear to be any difficulty in accounting 
for these phenomena. Mollusks could only find a habitat on the 
exterior portions of the reef; and it is evident that the eastern 
section of the limestone, more particularly, has been subjected to a 
considerable amount of denudation, so that the outer beds have, to 
a large extent, been removed. Quarrying operations have had the 
same result, since the contents of the limestone were first investi- 
gated, in considerable portions of the western section. Another 
noteworthy fact, and one which attracted the attention of Mr. 
Hennah, is that bivalves and univalves are rarely associated, but 
keep to their own distinct areas, wherein they are found in such 
abundance that the rock is at times completely crowded with them. 
Many of the paving slabs in Plymouth streets bear testimony of 
this. 
The limestone graduates into the slate on the south through 
calcareous shale, in which crinoidal remains abound. 
The whole of the limestone beds south are more or less fossili- 
ferous. Some of the shales, as at Staddiscombe, in lithological 
character and partially in contents, though their fauna is by com- 
parison very meagre, resemble the rocks of South Petherwin. 
The most abundant palsozoic fossils of the district include— 
Stromatopora polymorpha, Alveolites vermicularis, Acervularia 
pentagona, Cyathophyllum helianthoides, C. ccespitosum, Cysti- 
phyllum Damnoniense, Favosites Goldfussi, Smithia Hennahi, 
Actinocrinus tenuistratus, Fenestella antiqua, F. prisca, Retepora 
repisteria, Atrypa desquamata, A. reticularis, Merista plebeia, 
Orthis striatula, Rhynchonella cuboides, R. primiliparis, Spirifera 
simplex, Strigocephalus Burtini, Terebratula juvenis, Macrocheilus 
imbricatus, Murchisonia spinosa, &c. 
The bones of the mammoth, hippopotamus, rhinoceros tichorinus, 
or leptorhinus, cave lion, cave hysena, cave bear, ancient bear, the 
lesser bison, the long-fronted ox, the horse (fossil and plicated 
toothed), ass, various species of deer, wolf, fox, hog, and sheep, 
have occurred in the bone caves; and bones of a whale were found 
on the Hoe. 
The annexed list is offered, not as being exhaustive, but as being 
as nearly complete as circumstances have permitted, and as indi- 
