



Centimeters 



Figure 10. — 0. lurida from Pugpt Sound. Left (a) and 

 right (b) valves. Height 5.5 cm.; length 3 cm. 



0. equestris Say, horse oyster, crested oyster 

 (fig. 11). This small, noncommercial oyster of 

 the South Atlantic states. Gulf of Mexico, and 

 West Indies is often mistaken by laymen for 

 young C. virginica. Its average size is about 2 

 inches, but occasionally specimens measuring up 

 to 3K inches in height are found. Say (1834), 

 who described the species, hsts the following 

 identifying characters: Shell small, with trans- 

 verse wrinkles, and more or less deeply and 

 angularly folded longitudinally; ovate-triangular, 

 tinted with violaceous; lateral margins near the 

 hinge with from 6 to 12 denticulations of the 

 superior valve; superior valve depressed but 

 slightly folded; inferior valve convex, attached by 

 a portion of its surface, the margins elevated, 

 folds unequal, much more profound than those of 

 the superior valve; hinge very narrow, and curved 

 laterally and abruptly. 



The shape of the oyster shell is very variable 

 depending on crowding and type of substratum. 

 The most significant combuiation of features by 

 which 0. equestris can be distinguished from small 

 C. virginica and from 0. frons are as follows: a 

 rather higli vertical and crenulated margin of tlie 

 lower valve; off-center position of the adductor 

 nmscle scar; a dull greenish color of the interior 

 surface; and the presence of a single row of denti- 

 cles of the upper valve with the corresponding 

 depression on the lower valve (GaltsofF and Merrill, 

 1962). The number of denticles and their size 



very variable. Hanson (1960) does not include 

 0. equestris in the drawings of prodissoconchs 

 reproduced in his publication. A rough sketch of 

 the prodissoconch of 0. equestris, not showing the 

 details of hinge structure, is given by Menzel 

 (1953). 



This incubatory species frequently occurs in 

 large numbers on commercial oyster beds in associ- 

 ation with C. virginica. It thrives in waters of 

 high salinity (35 %o) but has been found in regions 

 where salinity is about 20 to 25°/oo. The north- 

 ernmost boundary of distribution established by 

 Merrill is about half way between Delaware and 

 Chesapeake Bays, (37°31' N., 73°18' W.) at the 

 depth of 60 fathoms. 



0. frons Linne (fig. 12). Small shells of 1 to 2 

 inches in height with radial plicated sculpture 

 and sharply folded margins. Valves closely set 

 with minute denticles almost around the entire 

 circumference. Muscle scar near the hinge. 

 Beaks slightly curved. Interior white, exterior 

 usually purple-red. Frequently attached to 

 branches or roots of mangrove trees by a series of 

 hooked projections. Connnon in Florida, Louisi- 

 ana, and the West Indies. Was found attached 

 to navigation buoys off Port Royal, S.C., and off 

 Miami Harbor, Fla. (personal communication of 

 A. S. Merrill). 



0. pennollis Sowerby (fig. 13). This small 

 oyster lives commensally, completely embedded 

 in sponges with only the margins of the valves 

 visible. The species is common along the west 

 coast of Florida, north of Tarpon Springs, where 

 it is constantly associated with tlie sponge Stellata 

 grnbi (fig. 14). Rarely exceeds 1.5 inches in size. 

 Surface of valves soft and silky. Beaks twisted 

 back into a strong spiral. Inner margins with 

 numerous small denticles. Nonedible species. 

 North Carolina to Florida and the West Indies. 

 Pycnodonte hyotis Linne. This species dredged 

 from 300 feet of water off Palm Beach was de- 

 scribed by McLean (1941) as 0. thomasi, nova 

 species, but identified by Ranson as Pycnodonte 

 hyotis. It is characterized by the peculiar foam- 

 like appearance of its shell structure, particularly 

 at the margins. Shells circular, 3 to 4 inches in 

 diameter. 



Fycnodonte is frequently found on navigation 

 buoys off Key West, Fla., and near the entrance 

 to Miami Harbor (personal communication of 

 A. S. Merrill). 



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