i64 REPORT OF NEW JERSEY STATE MUSEUM. 



Description. — Shell crown-like in shape, but passing into 

 cylinder. Radii extremely broad. Orifice large, nearly hex- 

 agonal. When operculum removed whole inside of cup- formed 

 shell ^•isible at once, as flat membranous base much smaller than 

 orifice. Under side of shell deeply concave. Outside of each 

 compartment formed by broad, rounded and somewhat promi- 

 nent though rarely divided ribs, as transverse ends of folded 

 walls. These ribs closely united together by finely serrated lines 

 of junction. Their outside surfaces delicately striated longi- 

 tudinally, plainly crossed by irregular transverse ridges, especi- 

 ally in lower part of shell. 



Scuta placed close together at rostral end of orifice, imbedded 

 in brownish tough longitudinally plicated horny substance, which 

 extends far beyond both ends of valves. In outline mitre-shaped, 

 or rounded and sub-triangular, a little curved, more or less 

 elongated, especially in young. Terga usually absent or rudi- 

 ment, as short thin plate of shell, barely visible to naked eye, 

 extending parallel and near to tergal edge of scutum. Lips of 

 aperture of sack prominent, highly so towards carinal end. As 

 shell not spreading much at base new folds in walls seldom 

 formed and external longitudinal ribs much more seldom 

 diA'ided. When new folds form, only one formed on one. as 

 rostral, side of each suture, instead of on both sides. Inner 

 ends of folded walls surrounding basal membrane narrow, 

 instead of almost scjuare. Lower edge of sheath, which pro- 

 jects freely, descends almost to level of basal edges of walls. 

 Outer ends of folded walls, forming transverse loops, in- 

 ternally filled up solidly by calcareous matter, instead of by 

 septa forming tubes. Radii little thicker in lower and outer 

 than in upper and inner part of each compartment. In middle 

 not reaching sheath by about half thickness of compartment, 

 consecpiently separated from plates in W'hich alse rest by large 

 chambers, which extend up nearly to apices of compartments. 

 Sinuous plates forming main portion of compound radii rather 

 thinner and closer together than in some species. Ake thick, 

 thickest medianly and there equal radii in thickness, almost 

 wedge-formed as lower edges very short compared with up])er. 



