356 PALAEONTOLOGY OF ILLINOIS. 



entrance to the anal aperture. The opening into this cavity is large, 

 somewhat broadly subtriangular, and there is a passage in a down- 

 ward course, which enters the inner body at the end of the oral 

 plate. 



The ambulacra are almost linear, rounded at the distal end ; they 

 rest deeply between the steep sides, the side-plates meeting the 

 sides of the sinus but are rarely touching them. The ambulacra 

 connect with the body through the lower side of the lancet-piece, 

 close to its middle line. The surface of the ambulacra is divided 

 by the food groove only, there being no other indented lines toward 

 the sides, such as seen in the Louisville species. The lancet piece 

 is entirely covered by the side pieces, it is lanceolate, and rests 

 with its proximal end, which is angular, between and partly upon 

 adjoining oral plates. The number of side pieces varies from 16 

 to each side in the smallest specimens to 22 or 23 in the largest 

 ones, each one with a socket, probably for the reception of a pin- 

 nule. There are also outer-side pieces but no hydrospire pores. 

 The oral aperture is pentagonal and comparatively large. 



The hydrospires are arranged in eight groups, of from 8 to 9 to 

 each set, and there are as many slits along the sloping sides of the 

 radial sinus. The two inner slits which are longer than any of the 

 rest, are concealed from view by the overhanging sides of the lan- 

 cet-piece. The succeeding ones are visible externally and decrease 

 in length with the decreasing width of the sides of the sinus. Not 

 all, however, decrease in that proportion, this is only the case with 

 the four or five outer ones, of which the outermost is but little 

 longer than wide ; the preceding three, from the fourth inward, are 

 nearly of equal length. It should also be remarked that the slits 

 have an irregular outline even in the best preserved specimens, and it 

 appears as if there had been pores in place of the slits. Each 

 slit opens into a sac. This, however, is so narrow, that the walls 

 almost touch each other, except at the lower end, where it attains 

 almost the width of the alternating folds of the hydrospires. There 

 is a great difference in the depth of the sacs; the two inner ones 

 extend into the body for a comparatively short distance, all suc- 

 ceeding ones increase their depth in tho same proportion as they 

 grow shorter upon the surface of the body. 



Remarks : Hetcroschisma gracile is in its form and general habits al- 

 most identical with Phoenoschisma Verneuilli Eth. and Carp, but it 

 has eight groups of hydrospires and not ten. It differs from both 



