A MONOGEAPH 



OF THJ5 



MOLLUSCA FROM THE GREAT OOLITE. 



PART II. 

 BIVALVES. 



Upon a general review of the Oolitic Lamellibranchiate Mollusks, it will be found 

 that a very large proportion consists of shells whose hinges may be arranged under 

 one or other of the following two groups, each of which has various generic modifications. 

 The first consists of a lengthened hinge plate, having a parallel series of transverse or 

 oblique teeth, as exemplified by Area, with its sub-genera Cucullcea, Nucula, Leda, 

 Macrodon, Isoarca, Limopsis. The second kind of hinge is altogether destitute of teeth, 

 and comprises the several genera of fossil Myadcs, as Pholadomya, &c., Mytilus with 

 Modiola, Lithodomus, Pinna, Trichites, and Thracia. Deducting these, together with 

 the forms whose hinge possesses only a ligamentary fossa, as Lima, Pecten, Hinnites, 

 Plicatula, and those in which the ligament is inserted in distinct pits, as Gervillia, 

 Perm, &c., it will be found that shells with hinge teeth constitute only a minority, and 

 that the great family of the Veneridm, though numerous with respect to individuals and 

 number of species, pertains only to few genera. Experience has led us to distrust many 

 generic names which have been given to these fossils, as PuUastra, Donax, Tellina, 

 Amphidemia, Chama, Lutraria, Sanrjuinolaria, Mactra, Gastrochcena, and Spondylus ; 

 Panopea is also a genus to which a very heterogeneous assemblage of testacea has been 

 referred ; Plagiostoma has by common consent fallen from the list of genera, the Oolitic 

 species being now referred to Lima. Nor has it in any one instance been ascertained 

 that any of the Oolitic bivalves have spoon-shaped processes corresponding to those of the 

 recent Mya and Lutraria. The shelly beds of the Great Oolite appear to have been 

 accumulated in a sea not sufficiently tranquil to become the habitat of the Myada ; the 

 entire family were gregareous, but in the shelly Oolite we rarely discover a single valve of 

 Arcomya, Ceromya, or of Homomya, the other genera of Myada being absent altogether. 

 The crypts of Lithodomus prove that genus to have existed in great profusion, although it 

 is very rare that the shells are found in the perforations themselves, neither can they be 



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