490 CRETACEOUS PALEONTOLOGY. 



1864. Lima pelagica Meek, Check List Inv. Foss. N. A., Cret 



and Jur., p. 7. 



1868. Radula pelagica Con., Cook's Geol. N. J., p. 724. 

 1886. Radula pelagica Whitf., Pal. N. J., vol. i (Monog. U. 



S. G. S., vol. 9), p. 61, pi. 9, figs. 3 and 5 (not fig. 4). 

 1905. Lima pelagica Johns., Proc. Acad. Nat. Sci. Phil. (1905), 



p. 12. 



Description. Shell, in large examples, attaining a height of 33 

 mm., a width of 25.5 mm., and a thickness of about 21.5 mm.; 

 oblique subovate in outline, the valves not gaping posteriorly. 

 The hinge-line straight, its length about one-third of the greatest 

 length of the shell, the hinge area of moderate height, with a 

 large central cartilage pit, hinge edentulous ; beaks at about the 

 center of the hinge-line, auriculations small, nearly equal. The 

 valves strongly convex and marked by about 25 strong, sub- 

 angular or rounded plications, and usually with a minute, ele- 

 vated rib in the bottom of each interspace; fine concentric lines 

 of growth, with sometimes an occasional stronger one, also mark 

 the entire surface of the shell. 



Remarks. This is a common member of the Navesink fauna, 

 and, besides its large size, it is characterized by the presence of 

 the minute secondary radiating ribs in the bottoms of the inter- 

 spaces between the primary plications of the shell, and by the 

 more or less subangular primary ribs. Whitfield included in 

 this species the shells described in the present report as Lima 

 zvhitfieldi, in which the secondary ribs are wanting and in which 

 both the plications and interspaces are rounder. In general form 

 the two shells are essentially identical, and in the internal casts, 

 the condition in which the species usually occurs, they probably 

 cannot be separated. The secondary ribs are more or less varia- 

 ble in the degree of their development in different individuals, and 

 are always more conspicuous on the anterior portion of the shell, 

 sometimes being faintly developed or entirely absent from the 

 central' and posterior portion. 



Morton would probably have included both of these forms un- 

 der his species R. pelagica, but the shells to which the name is 

 here restricted are by far the commonest forms of the genus in 



