86 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



triangular, and there is a passage in a downward course, which enters 

 the inner body at the end of the interradial plate. 



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

 (lee])ly between the steep sides, the side-plates meeting the sides of the 

 sinus at least toward the upjjer end of the ambulacrum. They 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-i)iece is entirely covered 

 by the side-pieces, it is lanceolate, and rests with its proximal end, 

 which is angular, between — and partly upon — adjoining interradials. 

 The number of side pieces varies from i6 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 pinnule. There are also outer-side 

 pieces, but no hydrospire pores. The oral aperture is jientagonal and 

 comparatively large. 



The hydrospires are arranged in eight grouj^s, 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 lancet-piece. 

 The succeeding ones are visible externally, and decrease in length with 

 the decreasing width of the sides along the sinus. Not all, however, de- 

 crease 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 ])ores 

 in place of the slits. Kach 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 hydro- 

 spires. There is a great difference in the dej^th of the sacs; the two 

 inner ones extend into the body for a comi^aratively short distance, all 

 succeeding ones increase their depth in the same i)roportion as they 

 grow shorter upon the surface of the body. 



Remarks: Hetcroschisma gracile is in its form and general habits 

 almost identical with Phceuoschisvia Vertieuilli Ether, and Carp., but it 

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

 HeteroschisDm altcnratutn and H. elongatuin, in that the summit of 

 the pyramids extend considerably above the oral crest.s, while in 



