LAMBLLIBEANCHIATA OF THE LOWER MARLS. 35 



The shells as found in the Cretaceous marls of the lower beds are 

 somewhat variable. There are, however, two very distinct varieties com- 

 mon. One agreeing with the above description (0. mesenterica Mort.), ex- 

 cept in the small number of denticulations assigned to the margins ; the 

 number being usually from seven to ten. The shell is small, not exceeding 

 half an inch in width and usually much less, while the shell is curved within 

 a circle of one and a quarter inches to one and a half inches in diameter. 

 The other variety, 0. nasuta Mort, being much larger, sometimes the valves 

 being nearly an inch in width and curved within a circle of about two 

 inches in diameter, while the denticulations vary from three to seven on the 

 outer side, counting the wrinkles of the hinge alations. The differences 

 between these two varieties are so great that were they living shells there 

 would be no hesitation in pronouncing them distinct species. 



In the Palaeontology of the United States Geological Survey of the 

 Territories (Vol. IX, p. 15), Mr. Meek has given descriptions of two spe- 

 cies of Ostrea from the Fox Hills group of the Western Cretaceous, under 

 the names 0. pellucida and O. subalata, separating the first-named species 

 from this one of the Eastern Cretaceous, on the want of "auricular append- 

 ages," a featui'e which I find quite common among the New Jersey speci- 

 mens; and on the less strongly denticulate margins of the shell, which is 

 an extremely variable feature, and the fact that the shell grows attached bv 

 the apex, more or less broadly, of the lower valve. While I write there 

 lies before me two New Jersey shells which have grown together, attached 

 to each other for their entire length, showing that this species also seems to 

 have had that same habit to a certain extent. The second one of Mr. Meek's 

 species, 0. subalaia, is so nearly like the variety nasuta Mort., that it is 

 very difficult to see wherein lies the specific distinction, more especially 

 as the one before me possesses the alation on the left side only, with beaks 

 exactly parallel to those of his figure given, but is coupled with five very 

 strong denticulations on the outer margin. On examination of the form as 

 it occurs in the Eocene at various places in this country the same variations 

 are detected, although the coarser denticulated variety is less common and 

 not so extreme; and among the European specimens the same changes are 

 visible. In the form given in Sowerby's Min. Conch., t. 135, Fig. 1, as 



