534 Prof, Sedgwick on the [May, 



Casts, probably derived from the same shell, are found in some 

 earthy beds between the last mentioned locality and Brook 

 Point. 



(6.) To this list may be added the casts of at least three or 

 four species of univalves which abound in the upper part of the 

 cliff near Shanklin. 



Every one who has examined the localities of fossils must have 

 remarked that in strata abounding in the oxide of iron, they are 

 frequently stripped of their shelly covering. In soft argillaceous 

 beds, even the colours of the original shell are sometimes pre- 

 served, but the specimens are generally friable, and often disfi- 

 gured by compression. In other mineral masses, more espe- 

 cially such as contain a considerable proportion of carbonate of 

 lime, the fossils which can be detached from the matrix are in a 

 beautiful state of preservation. Several species which exist only 

 as casts in the ferruginous cliffs of Shanklin, are finely preserved 

 in Red-clift' reef. It is probable that perfect shells belonging to 

 all those species of which we have now only the casts, may be 

 found by any one who has time to examine the localities above- 

 mentioned. 



4. Bivalves. — (1.) Gryphota siuuata. Min. Con. t. 336. 



(2.) Ostrea. At least three distinct species. 



(a.) A palmated cockscomb oyster, associated with, and 

 sometimes adhering to, the preceding species. They are found 

 in regular beds which traverse the cliffs of Blackgang Chine and 

 Shanklin Chine. Traces of the same beds appear to the west of 

 Brook Point. . 



(b.) An undescribed species ; much elongated ; in general 

 form somewhat resembling the ostrea tenera. Min. Con. pi. 252, 

 p. 2, 3. This species abounds in some of the calcareous beds to 

 the west of Brook Point. 



(c.) A small flat oyster, associated with the preceding. Thin 

 beds of this fossil traverse some of the argillaceous strata of San- 

 down Bay. The shelly matter is sometimes replaced by minute 

 crystals of selenite which have originated in its destruction. 

 The analogous facts exhibited by the Oxford clay and the Kim- 

 meridge clay, are too well known to require any description. 

 There are probably more species of this genus ; but the speci- 

 mens are generally too ruinous to show any good distinctive 

 characters. 



(3.) Ferna. 



(fl.) A large thick quadrangular shell. 



(b.) ResembHng per?ia aviculoides (Min. Con. pi. 66), but the 

 fragments too imperfect to determine the species. Both are 

 found in Red-cliff reef. The perjta aviculoides is a characteristic 

 shell of the middle oolite formation {coral rag). A perna of rude 

 trapezoidal form is also found in that formation near Weymouth, 

 but is of an entirely different species from the quadrangular shell 

 above mentioned. ~ 



