276 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



concentric [lines], crossed by rather coarse, radiating-, bifurcating striae, 

 and usually four prominent folds, which commence on the umbo at some 

 distance from the beak and terminate at the angles of the border. 



" In nearly all full-grown specimens that I have seen the radiating 

 striae occupy merely the rostral half of the shell. The dimensions of an 

 average specimen are: Length, 1.30 inches; width, 0.80; thickness, 0.40." 



This species is much more arcuate than is usual in the typical section 

 of this genus, in which respect it resembles some forms of Alectryonia; 

 but it wants the strongly plicated surface and zigzag margins which 

 characterize that subgenus. It doubtless belongs to the typical section 

 of Ostrea, although a somewhat aberrant form. It is probable that this 

 species is identical with O. crenulo margo Eoemer (Kreidebildung von 

 Texas, p. 76, Taf. ix, fig. 6 a, b) ; but as Dr. Eoemer seems to have 

 obtained examples of the upper free valve only, we are left in some 

 doubt upon this point. In none of the somewhat numerous examples 

 sent by Professor Mudge to the office of the Survey from Denison, 

 ^Northern Texas, does the upper valve exhibit any plication of the sur- 

 face, or possess the prolongations from the free convex margin, which 

 are so characteristic of the under valve of the species. Dr. Bhumard's 

 figure of the large upper valve, however, shown on plate 8, fig. 3b, shows 

 incipient prolongations of its margin, but its lines of increment indicate 

 that they were not developed until the shell approached maturity. 

 These upper valves, although presenting the variability common to the 

 family, possess all the essential specific characters that are shown by 

 the upper valve of O. crenulo-margo, as figured and described by Dr. 

 Eoemer. It should be remarked, however, that the collection contains 

 no example with both valves in situ; also, that the free margins of the 

 under valve, except those portions between the middle and the beak, 

 show little or notliing of the fine crenulation that marks the entire 

 margin of the upper valve, which character suggested the specific name 

 to Dr. Eoemer. 



The association of these two valves in the same layers, with no others 

 that could possibly belong together, and the fact that the four long- 

 marginal processes of the under valve were not developed until the shell 

 approached maturity, together with their common characteristics, seem 

 to prove, beyond question, that they belong to one and the same species. 

 Dr. Shumard's examples, illustrated on plate 8, were obtained from the 

 bluffs of Eed Eiver, Lamar County, Texas ; Dr. Eoemer's, from Fred- 

 ericksburg, Tex., and Professor Mudge's, from Denison, Tex.; the lat- 

 ter furnishing those which are illustrated on plate 5. 



Ostkea (Alectryonia) bellaplicata Shumard. 



Plate 4, figs. 3 a, 1) ; and plate 8, figs. 2 a and &. 



Ostrea 'bdla'plicata Shtjm., I860, Trans. St. Louis Acad. Sci. vol. 1, p. 608. 



This species is more robust than is usual in the subgenus Alectryonia, 

 but the strongly plicated surface, zigzag character of the free margins, 

 moderately extended hinge-line, and subalation of the postero-dorsal 

 portion are characters which agree with those of that subgenus, and 

 separate it from the typical forms of Ostrea. The following is the origi- 

 nal description of the species by Dr. Shumard (loc. tit.)*. 



" Shell of medium size, ovate or subcircular, anal and pallial borders 

 rounded ; buccal border subtruncate ; valves unequal. Superior valve 

 usually flat, but sometimes concave, or even gently convex ; hinge mar- 

 gin oblique^ nearly straight ; beak obtusely angular, angle from 105° to 



