648 CRETACEOUS PALEONTOLOGY. 



evidently reversed the anterior and posterior extremities of the 

 shell. It is not possible to determine from the specimen to what 

 extent the shell was gaping, but it appears to have been nearly 

 or quite closed in front and gaping behind. The species differs 

 from P. decisa of the lower beds, in its more slender form and 

 more rounded extremities, especially the posterior, which lacks 

 entirely the oblique truncation. 



Formation and locality. Manasquan marl, near New Egypt 

 (Whitfield), Medford. 



Geographic distribution. New Jersey. 



Family GASTROCHAENIDAB. 



Genus GASTROCHAENA Spengler. 



Gastrochaena whitfieldi n. sp. 



Plate LXXIIL, Figs. 10-12. 



Description. Diameter of the tube 14.5 mm. at its larger, 

 closed extremity, decreasing to n mm. in a distance of 22 mm. 

 Shell broadly gaping ventrally and posteriorly, the beaks anterior ; 

 the dimensions of the type specimen are : length, 16 mm. ; height, 

 8.5 mm. Valves somewhat twisted, subtriangular in outline, 

 truncated behind and pointed in front, but with the anterior, 

 pointed extremity inflected almost at a right angle so that in 

 lateral view the valves appear to be quadrangular, the dorsal 

 margin gently convex posteriorly. A low, obtusely angular, very 

 oblique umbonal ridge extends from the beak to the postero-basal 

 angle, below this ridge the surface is slightly concave to the ven- 

 tral margin, above it is convex nearly to the dorsal margin where 

 it becomes a little concave. Surface of the shell marked by fine, 

 concentric lines of growth which bend upward abruptly at the 

 oblique umbonal ridge; upon the ventral region and especially 

 upon the inflected anterior extremities of the valves, the markings 

 become somewhat sublamellose. 



Remarks. This species is based upon a single very perfect 

 cast of a complete shell, with a partial impression of the external 

 surface showing the surface markings. A cast of the inner por- 



