2 MOLLUSCA FROM THE GREAT OOLITE. 



detected in the substance of the valves of Trichites, although few shells of that genus, or 

 of the adult specimens of Crassina, can be found, which are not bored or even honey- 

 combed by their perforations, a fact which should teach us that the numbers of fossil 

 specimens do not afford in every instance a sure indication of their former actual numbers. 

 The Trigonice, which hold so important a position in the Oolitic testacea, are represented 

 in the shelly beds by a great profusion of individuals ; nevertheless, these beds do not 

 appear to have possessed conditions favorable to the development of the several species ; 

 three occur abundantly in their earliest stage of growth, but in proportion as they 

 increase in size, their numbers diminish, so that adult specimens are comparatively rare. 

 In the species referred to Pteroperna (a sub-genus of Avicula), will be found some 

 interesting forms serving to connect Avicula and Pterinea with the Polyodontce, the hinge 

 being somewhat identical with that of Macrodon ; it occupies a conspicuous position in the 

 Oolitic system. Another new generic form which remains to be exemplified is 

 Tancredia, [Hettangia, Terquem ;) although of small dimensions, and destitute of ornament 

 or remarkable figure, it is nevertheless the genus which, by the constantly recurring 

 force of numbers, most strongly dwells upon the recollection of those who have with their 

 own hands cleaved the shelly beds of the Great Oolite. 



On the other hand, in the muddy deposits which are associated with the Great Oolite, 

 the family of Myada were tolerably abundant, if we may judge from the numerous species 

 of Pholadomya, Panopoea, and its allied genera, Pleuromya, Arcomya, Homomya, which are 

 found in the beds of indurated marl, intercalated with or overlying some portions of the 

 shelly Oolite, throughout an extensive area; whilst many of the shells, the Pholadomya 

 especially, retain the normal position in which they appear to have lived. 



EivALViA, Linn. — Lamellibrajjchiata, Blainv. 



OsTREA, Linnaus, 1758. 



General Character. Shell adherent, inequivalve, foliaceous, irregular ; umbones sepa- 

 rated, slightly diverging ; ligament internal, placed in a deep grooved trigonal pit, beneath 

 the umbones. Muscular impression nearly central. 



OsTREA RUGOSA, Goldf. Tab. I, fig. 4. 



OsTREA RUGOSA, Goldf. Petrcf., tab. 72, fig. 10. 



Testa ovatd, valvd inferiore profunda concentrice rugosd, margine inferiore plicate ; 

 superiore subconvexd, undulatd-rugosd. 



Shell ovate ; inferior valve deep, with concentric rugose plications, the lower margin 

 plicated ; the upper valve slightly convex, rugose, and undulated. 



The typical form of this shell, which we have provisionally referred to 0. rugosa, 

 Goldf., has a considerable resemblance to 0. acuminata; but the attached surface is 



