Maryland Geological Survey 513 



oblique to the surface. It has been confused in the synonomies and col- 

 lections with Nucula percrassa Conrad of the South Atlantic and Gulf 

 states. Nucula percrassa has been reported from New Jersey by a num- 

 ber of authors including Gabb, Meek, Whitfield, and Weller, but mostly in 

 the form of casts. In Maryland, the shell itself is well preserved and 

 abundant. The comparison of a large series from Maryland with a series 

 from Owl Creek, 3 miles north of Ripley. Mississippi, Conrad's type 

 locality, makes it probable that the forms are distinct. Nucula slackiana 

 Gabb runs higher and heavier than the Nucula percrassa of Conrad; a 

 typical individual from Maryland measures 33.8 mm. in length and 

 23 mm. in altitude, while one from Ripley measures 25.3 mm. in longi- 

 tude, and 19.5 in altitude. The umbones are less prominent in the 

 northern species, less convex, and more feebly opisthogyrate, the lunule 

 and escutcheon less sharply differentiated and the ventral margin less 

 flattened. There is a strong tendency in the N. percrassa toward a slight 

 contraction of the ventral margin toward the posterior margin, thus giving 

 to the rear end of the shell a nasute aspect which is absent in N. slackiana. 

 The angle of divergence between the anterior and posterior series of hinge 

 teeth is higher in Conrad's species, the muscle scars are usually deeper and 

 the crenulations upon the inner ventral margin finer, sharper, and farther 

 produced into the interior of the valve. 



The northern race is much more uniform in its characters, however, 

 than the southern. Conrad's type is rather extreme. Many of the indi- 

 viduals from the Ripley are very much higher, relatively, and though they 

 do not quite bridge the gap between N. percrassa and N. slackiana, they 

 materially diminish it. There is, however, a peculiar characteristic 

 flattening of the umbones and of the ventral margin of the Gulf forms 

 which is, apparently, not reproduced in any of the northern specimens. 



Occurrence. — Matawan Formation. Summit Bridge, Chesapeake 

 and Delaware Canal, Delaware. Monmouth Formation. Two miles 

 west of Delaware City on John Higgins farm, Delaware. Brightseat, rail- 

 road cut west of Seat Pleasant, Brooks estate near Seat Pleasant, Friendly, 

 1 mile west of Friendly, and McNeys Corners, Prince George's County, 

 Maryland. 



