342 PALEONTOLOGY OF NEW YORK. 



enlarging, with the dorsal side concave, and the ventral and lateral faces 

 convex. Apical angle about 25°. The apical portion is rather more atten- 

 uate than the succeeding part of the tube, on account of the sides of the tube 

 being concave for a short distance. 



Cbamber of habitation large, gibbous, having a length equal to the ventro- 

 dorsal diameter of the tube at the last septum, or apparently nearly one- 

 fourth of the entire length of the shell. Dorsal side nearly straight, and 

 parallel to the longitudinal axis. Ventral side very convex, and sloping 

 rapidly to the aperture. Crennlated zone not distinctly marked, but indi- 

 cated by irregular, obscure furrows, which are continued over the cast of the 

 walls of the air-chambers. 



Aperture slightly expanded, large, oval, having a width of more than the 

 greatest ventro-dorsal diameter of the tube. Small aperture present, but not 

 fully observed. 



Air-chambers regularly increasing in depth from the apex to the grand 

 chamber, varying from six to eight mm. in the length of fifty-five mm. The 

 chambers are somewhat shallower on the dorsal side, owing to the curvature 

 of the tube. Septa thin, smooth, having a concavity greater than the depth 

 of the air-chambers. Sutures straight and horizontal. 



Siphuncle moniliform, near to the ventral side, abruptly expanding in the 

 cavities of the air-chambers to a diameter equal to twice its diameter at 

 the septa, or equal to the depth of the chambers, which is about one-sixth 

 the diameter of the tube. The abrupt expansion of the siphuncular tube 

 produces a small areola on the septa, on account of its contact with the sur- 

 face. Near the apex the distance from the centre of the siphuncle to the 

 chamber walls is four mm. Toward the outer chamber it gradually becomes 

 more distant from the septal margins, and at the last septum is nine mm. 

 from the exterior walls of the tube. 



The test has a thickness of about 1.5 mm. on the chamber of habitation. 

 The surface of the nacreous layer is marked by very fine, longitudinal striae 

 and irregular, concentric lines. The surface of the epidermal layer shows 

 fine, lamellose lines of growth, and indistinct longitudinal striae. Both layers 



