522 bulletin: museuim of comparative zoology. 



are quite barren of fossils. Next in descent is a highly fossiliferous 

 calcareous shale, from which slabs were dug that literally teemed with 

 fossil shells. Several of these slabs were obtained, and although 

 Rafinesquina is well represented on them, Strophomena is absent with 

 the single exception of a pedicle-valve of Strophomena jplanodorsata. 



Strophomena abscissa, sp. nov. 

 Plate 1, fig. 6. 



Shell small, roughly triangular in outline. Hinge-line straight; 

 cardinal angles often mucronate, especially in the younger specimens. 

 Greatest height of the cardinal area varies from one to two milli- 

 meters. The surface of the brachial -valve is abruptly deflected from 

 three to six millimeters posterior to the margin, the angle formed 

 measiuing 90°-105°. This sudden deflection is a highly characteristic 

 feature of this form and makes confusion with any other Maquoketa 

 species impossible. 



The pedicle-valve is weakly convex at the beak, then turns and 

 follows the shape of the brachial-shell, producing a cup-like cavity, 

 which lacks the sharp angles of the brachial -valves. 



Striae are distinctly alternating on the brachial -valve, from three 

 to nine finer between two coarser ones. The coarser striae become 

 more pronounced upon the vertical part of the brachial -valve; the 

 finer intercalated ones tend to become obscure on this area, with the 

 exception of one to three in each series which become coarse. 



This makes the deflected anterior region of the shell quite distinctly 

 striated and sharply contrasts it with the finely marked posterior part. 



Specimens vary little in size, a typical one measuring 15 mm. along 

 the hinge-line, 10 mm. in length, 5 mm. in depth. 



Locality: — Lower Maquoketa at Clermont, low^a. Holotvpe, 

 M. C. Z. 8,540. 



Strophomena laminata, sp. nov. 

 Plate 1, fig. 9, 10. 



This form is smaller than the average for the genus, and less coarsely 

 striated. It resembles no other species in this fauna so closely that 

 confusion might result, for it possesses a characteristically laminated 

 brachial-valve which is unique. 



Shell semiovate in outline; slightly wider than long, with the 



