630 SYSTEMATIC PALEONTOLOGY 



able regions where the struggle for existence is less keen and they do not 

 have to compete with more virile groups. The few recent forms, including 

 the generic type, P. Candida Sowerby, inhabit the ocean depths, some of 

 them below the one thousand fathoms line. A single species has been 

 recorded off the Japan coast, one from off the Africa coast and the rest 

 from the Antillean region. 



A. Outline subcylindrical ; umbones broad, rounded, not conspicuously 



high Pholadomya occidentalis 



B. Outline ovate-trigonal; umbones relatively narrow, subangulated, con- 



spicuously high Pholadomya conradi 



PHOLADOMYA OCCIDENTALIS Morton 

 Plate XXXVII, Figs. 1-3 



Pholadomya occidentalis Morton, 1833, Am. Jour. Sci., 1st ser., vol. xxiii, 



p. 292, pi. viii, fig. 3. 

 Pholadomya occidentalis Morton, 1834, Syn. Org. Rem. Cret. Group, U. S., 



p. 68, pi. viii, fig. 3. 

 Pholadomya occidentalis Meek, 1864, Check List Inv. Fossils, N. A., Cret. 



and Jur., p. 14 (ex parte). 



Pholadomya occidentalis Conrad, 1868, Cook's Geol. of New Jersey, p. 727. 

 Pholadomya occidentalis Whitfield, 1885, Mon. U. S. Geol. Survey, vol. ix, 



p. 175, pi. xxiv, figs. 1-3. 



Pholadomya occidentalis Johnson, 1905, Proc. Acad. Nat. Sci., Phila., p. 13. 

 Pholadomya occidentalis Weller, 1907, Geol. Survey of New Jersey, Pal., 



vol. iv, p. 513, pi. Ivi, figs. 1-3. (Synonymy excluded.) 



Description. " Oblong-angular, ventricose near the beaks ; with 

 twenty-five to thirty narrow, elevated, subtortuous costae, having broad, 

 slightly concave intervening spaces. Length 2 inches, breadth 3 inches. 

 An extremely variable species. I possess five specimens (all more or less 

 broken), in all of which there is a difference in the number and relative 

 position of the ribs." Morton, 1833. 



Type Locality. Chesapeake and Delaware Canal. 



" The dimensions of an average-sized specimen are : Length about 

 70 mm., height 47 mm., thickness 45 mm. Shell subovate or sub- 

 elliptical in lateral outline, and cordate from in front. Hinge line 

 straight, about two-thirds as long as the shell ; anterior margin rounding 

 from the cardinal extremity into the basal margin, or obliquely subtrun- 



