286 > NEW YORK STATE MUSEUM 



backward till it meets the cruralium. This ridge gives off two 

 lateral branches from its middle portion, equally raised but narrower 

 and pointing toward the ends of the transverse axis. These ridges 

 form the inner boundaries of four deep muscular pits of nearly equal 

 size. The posterior pits with a very slight additional extension back- 

 ward would leave the cruralium as a narrow platform supported by 

 the wider portion of the median ridge. The two anterior pits are a 

 little nearer together and each shows three distinct muscular impres- 

 sions separated by two very narrow and slightly raised ridges. The 

 middle scar of each three is the largest, is subtriangular, and has its 

 apex pointing a little inside of the well marked dental sockets ; the 

 outer pair are a little smaller, of nearly the same shape and with apex 

 pointing very nearly toward the small, narrow cardinal process ; the 

 outer impression of the three is a little smaller still and rather 

 rounded in outline. The pedicle valve bears a wide spondylium well 

 raised from the valve and supported by a fine and narrow median 

 septum which is continued anteriorly to the middle of the valve. 

 The arrangement of the genital and pallial sinuses is shown in plate 

 5, figures lo, 14. The muscular areas on the spondylium are not dis- 

 tinctly separated but one can distinguish three tracts, a central and 

 two outer of nearly the same area, the boundaries of which are not 

 sharply limited. The delthyrium is bordered by a narrow raised 

 ridge which is continued around the cruralium of the brachial valve. 

 At the apex this well rounded border meets a straight raised ridge 

 tangent to the curve, and just anterior to the ends of this ridge, and 

 outside of the curved border, are two short, narrow, depressed pits 

 usually worn off in most of the valves found. 



LAMELLIBRANCHIATA 

 Genus modiolopsis Hall 

 Modiolopsis subquadrilateralis sp. nov. 



Plate 4, figures 8, 9 



Description. Shell small, from anterior to posterior extremity 

 nearly 9mm. Rather elongate ovate with anterior margin truncate, 

 the straight portion of this margin making an angle of about 125° 

 with the anterior third of the dorsal margin which is also straight; 



