66 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



"Length of an example rather under the average size, from umbo to 

 basal margin, about 1 dm ; breadth, 8 cm ; convexity of the large valve, 

 nearly 6 cm . 



"Among the numerous examples of this species in the collections, 

 none, except the one figured, show the radiating costae, and these costae 

 seem to be quite different from those, at least of the typical forms, of 

 E. costata Say. Mr. Conrad states, however, that in New Jersey, Ala- 

 bama, and Texas ever y intermediate gradation of form and character is 

 found, from typical forms of E. costata to E.ponderosa. Judging from our 

 examples alone, no person would suspect such specific relationship ; 

 and, in want of any intermediate forms for personal examination, I 

 prefer at present to place our examples under the designation giv.en by 

 Dr. Roemer. 



" Position and locality. — Strata of the Cretaceous period; east of Im- 

 practicable ridge, Utah." 



The above is Dr. White's description of the Utah fossils which do 

 not differ in any respect from examples collected at Roemer's typical 

 locality. Mr. R. T. Hill has shown that in Texas the typical Exogyra 

 costata is characteristic of the uppermost beds of the Cretaceous, while 

 E. pondcrosa occurs at a lower horizon, ranging down as low as the 

 Austin limestone. My observations in Texas and several other south- 

 ern states have confirmed this fact. It is therefore better as a matter 

 of convenience and for use in geologic work to keep these varieties 

 under separate specific names. 



ANOMIID^E. 



Genus ANOMIA Linnaeus. 



Anomia subquadrata n. sp. 



PI. viii, Figs. 8 and 9. 



Upper valve subquadrate in outline, thin and pearly, varying greatly 

 in convexity; cardinal margin straight, or very slightly arched, about 

 half as long as the greatest anteroposterior diameter of the shell; pos- 

 terior margin nearly straight in the middle and rather abruptly rounded 

 above and below to join the cardinal and ventral margins respectively; 

 ventral and anterior margins forming a continuous curve, which is more 

 narrowly rounded in the middle ; beak inconspicuous, submarginal. Sur- 

 face marked by obscure, concentric undulations and lines of growth, 

 and very faint traces of radiating striae. 



The figured types have the following dimensions: Height, 20 and 

 23 mm ; transverse diameter, 22 and 23 m,n ; convexity, 6 and 3 mm . 



This species has some resemblance to Anomia nitida Meek. 1 It is 

 possible that they may prove to be identical when good material for 



1 Macomb's Expl. Exped. from Santa Fe to junction of Grand and Green rivers, Geol. Kept., p. 125, 

 pi. 1, Figs. 4a and &. 



