PEBBLE PHOSPHATE DEPOSITS. 
75 
Phosphatized casts of invertebrates are rather numerous in the 
phosphate beds. Most, if not all of these, however, are washed in, 
as previously stated, from the bed rock beneath. Among the in¬ 
vertebrates in the phosphate beds, which seem to be contemporaneous 
with the deposits, are silicified oysters found in the pit of the Pem¬ 
broke mine of the Coronet Phosphate Company, where they occur 
in some abundance making up a well marked stratum within about 
two feet of the top of the workable phosphate bed. Dr. W. H. Dali 
of the U. S. National Museum who has kindly examined specimens 
of this oyster writes that it is probably Ostrea mauriciensis Gabb, 
differing from the ordinary 0 . virginica by the deep pit under the 
hinge line.* The two species, however, are very closely related. 
Fig. 9.—Side view of land tortoise. Same specimen as figure 7. One-twelfth 
natural size. The part restored is indicated by dotted lines. 
If this oyster is 0 . mauriciensis it would seem that it must have 
been washed into this formation from the Alum Bluff formation. 
However, from the fact that the oysters occur in abundance, and in 
some cases in large masses near the top of the workable phosphate 
bed, it seems probable that they actually belong to the Bone Valley 
formation. 0 . mauriciensis, it may be noted (Dali, Bull. 90, p. 123, 
U. S. Nat. Mus.) is itself a species of doubtful validity. 
The fossils that have been obtained from the formation indicate 
that the land pebble phosphate deposits were accumulated during 
either late Miocene or early Pliocene time. 
*Letter of May 14, 1915. 
