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



described by Boemer. They also differ in having the under valve 

 always marked by regular radiating costse, while that of E. Iceviuscula 

 is generally quite smooth, or rarely presents traces of nearly obsolete, 

 rather broad, plications, as represented by Fig. 3c of Roemer's PI. ix 

 (Kreid. von Texas). It seems likewise to be a thinner and less robust 

 shell than Koemer's species, and holds a lower strati graphical position. 

 In surface markings, as well as in general form, it closely resembles 

 young specimens of E. columba of Lamarck (Aniin. sans Vert., vi, 398), 

 as figured by Goldfuss in his Pctrefact. Germ., and by d'Orbigny in 

 the Paleont. Francais. It never attains more than one-eighth the size 

 of adult individuals of that species, however, and differs in having an 

 oval instead of a circular upper valve." 



Locality and position. — From the Lower Cretaceous of Dr. Newberry's 

 New Mexican section at Covero, New Mexico, and from the base of the 

 Middle Cretaceous of the same section at Galisteo, and in the Sierra 

 Abajo. The specimens described by Dr. White came from the east 

 bank of Eio Puerco, 6 miles below Casa Salazan, New Mexico, and it 

 has also been collected in the Lower Cross Timber sands and in the 

 Eagle Ford shales of Texas. 



Exogyra l^evitjscula Roemer. 



PI. vin, Figs. 5 and 6. 



Exogyra Iceviuscula Roemer, 1852, Kreidebildnngen yon Texas, p. 70, PI. 9, Figs. 3a-c; 



Conrad, 1857, U. S. & Mex. Bound. Sur., p. 154, PI. 7, Figs, la, b; White, 1876, U. 



S. Geog. &, Geol. Sur. West 100th Meridian, p. 173, PI. 17, Figs. 2a-d; 1884, 4th 



Ann. Kept. U. S. Geol. Sur., p. 305, PI. 52, Figs. 3, 4, 5. 

 Ostrea ferdinandi Coquand, 1869, Monog. Gen. Ostrea, Terr. Cre"t., p. 33. 

 Exogyra ponderosa Hill, 1889, Check List Cret. Invert. Fossils of Texas, p. 6. 



" Shell of moderate size, capacious, somewhat semiovate in form, 

 suborbicular in marginal outline ; test not massive; larger valve much 

 inflated and subhemispherical; a very indistinctly defined umbonal 

 ridge is to be seen upon some examples, especially near the beak, but 

 in others this feature is wanting. 



"Umbo small, distinctly spiral, making about two volutions, some- 

 times nearly free, but often very closely curved, giving the posterior 

 side an umbilicated character, sometimes having a very small scar of 

 attachment, but often without such a scar, and always quite symmet- 

 rical, or at least not distorted^ as the beak often is in other species of 

 this genus; periphery of its curve usually extending beyond the hinge 

 line, but sometimes not. The smaller valve is nearly flat, or slightly 

 and somewhat irregularly concave, suborbicular in outline. Surface 

 of both valves having a smooth aspect, but it is marked with such 

 lines of growth as are common to other genera of shells, and free from 

 the lamination of surface so common in the Ostreida?. 



"Diameter of the largest example in the collection, from umbo to 

 basal margin, 47"" n ; transverse diameter, 42 ,m "; depth of the larger 

 valve, 2S mm , 



