706 SYSTEMATIC PALEONTOLOGY 



becoming obsolete near the center of the shell. Surface of the cast appar- 

 ently smooth." WelJer, 1907. 



Type Locality. Middletown, New Jersey. 



Leptosolen elongata is much smaller than Leptosolen biplicata Conrad. 

 The casts the only form in which the species is definitely known are 

 readily separable from those of the latter by the posteriorly inclined rather 

 than the vertical sulcus produced by the internal rib. 



Occurrence. MONMOUTH FORMATION. Brightseat, Prince George's 

 County. 



Collections. Maryland Geological Survey, New Jersey Geological 

 Survey. 



Outside Distribution. Monmouth Formation. Eed Bank sand, New 

 Jersey. 



Superfamily MACTRACEA 

 Family MACTRIDAE 



Genus SPISULA Gray 

 [Mag. Nat. Hist, n. s. vol. i, 1838, p. 372] 



Type. Mactra solida Linne. 



" Shell small, subequilateral, trigonal, with a thin epidermis, adjacent 

 beaks and concentrically grooved dorsal areas ; pallial sinus small, rounded ; 

 gape obsolete ; valves convex ; ligament sagittate, set in a callous area close 

 to the dorsal margin and not set off from the chondrophore by any shelly 

 ridge ; dental armature normal, strong, not concentrated ; the opposed sur- 

 faces of the laterals transversely grooved ; left cardinal small, prominent, 

 with a small posterior accessory lamella, the posterior ends of both pro- 

 jecting over the chondrophore; right cardinal with the arms coalescent 

 above, the anterior arm close to the dorsal shell-margin ; hinge plate thick 

 and flattish; exterior smooth or concentrically striated; the dorsal areas 

 ill-defined." Ball, 1898. 1 



The absence of a shelly lamina between the chondrophore and the liga- 

 ment separates Spisula from Mactra. Furthermore, the laterals of the 



Etymology: Spissus, thick. 



1 Trans. Wagner Free Inst. Sci., Phila., vol. iii, pt. iv, p. 878. 



