342 
PALAEONTOLOGY 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. 
Chamber 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. Crenulated 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-live 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 
