MOLLUSCA. 371 



vation. In the Marshalltown marl near Swedesboro, the shell sub- 

 stance is perfectly preserved, but in other localities the specimens 

 are usually more or less modified internal casts and externaj im- 

 pressions. Only rarely is an internal cast met with which pre- 

 serves well the muscular impressions and other markings of the 

 interior of the shell. In most cases, after the dissolution of the* 

 shells, the soft material in which they were buried has been com- 

 pressed, closing the cavities left by the shells and obliterating the 

 natural markings of the casts. 



Whitfield considered the two species, N. percrassa and AT. 

 slackiana as distinct, but a careful study of a large number of 

 specimens from New Jersey, and a comparison of them with 

 southern representatives, has shown that it is absolutely im- 

 possible to draw any lines which can be used to distinguish the 

 two forms. Gabb himself, the author of the species N. slackiana 

 gave expression to the same conclusion in 1876. 



A careful examination of the type of Donax fordii Con., in 

 the collection of the Philadelphia Academy of Science, shows it 

 to be only an imperfect and somewhat distorted example of 

 Nucula percrassa. 



Formation and locality. Cliffwood clay, Clifrwood Point 

 (105) ; Merchantville clay-marl, near Jamesburg (141), Lenola 

 (163), Merchantville (162); Woodbury clay, Lorillard (102),. 

 near Haddonfield (183) ; Marshalltown clay-marl, near Swedes- 

 boro (177) ; Wenonah sand, near Marlboro (130). 



Geographic distribution. New Jersey, Maryland, North Caro- 

 lina, Alabama, Mississippi and Texas. 



Nucula whitfieldi n. sp. 

 Plate XXIX., Figs. 6-12. 



Description. Shell triangularly subovate in outline, with 

 moderately convex valves ; the beaks pointed, situated from one- 

 fifth to one-fourth the length of the shell from the anterior ex- 

 tremity; postero-dorsal margin moderately convex from the 

 beak to the somewhat sharply rounded posterior extremity of 

 the shell which is below the mid-height; ventral margin convex 

 throughout, curving upward more rapidly in front than behind* 



