1888.] NATURAL SCIENCES OF PHILADELPHIA. 345 



of the rays are broken off a little above the first bifurcation, so that 

 the whole structure is plainly visible, and, except in one or two 

 places, is in the most perfect condition (PI. XVIII. figs. 11 and 11). 



The ventral surface is covered by an integument of very small^ 

 irregular plates, attached to some larger plates within the dorsal cup, 

 and the marginal plates along the free rays, forming in connection 

 with the latter along the rays pouches or sacs which extend far up 

 along the arms, being traced in other specimens to the second and 

 third bifurcation. Along the median radial portions of this integu- 

 ment rest the ambulacra, which pass from the middle of the disk to 

 the rays, following their bifurcations. There are two rows of sub- 

 ambulacral pieces, transversely elongate and alternately arranged, 

 forming the floor of the groove. The groove is bordered by side 

 pieces, and roofed over by two rows of interlocking covering plates, 

 which seem to have been moveable, as they are open in several places 

 in the specimen, — indeed they appear to be mostly in that condition. 

 The anterior ambulacrum is perfect, with the covering pieces in place, 

 and slightly gaping. In the right antero-lateral ambulacrum the 

 covering plates and side pieces have slipped off from the subambu- 

 lacral plates, and lie interradially to the left of them, but are other- 

 wise not much disturbed. In the other three ambulacra the covering 

 pieces for the most part are gone, leaving only the floor with the 

 subambulacral plates in position. The plates covering the interpal- 

 mar areas are also well shown in the specimen at three sides; at the 

 two others the integument is not intact, and the plates lie scattered 

 upon the surface. When one sees the exceedingly frail character of 

 this covering, he may well wonder at the exceptional good fortune 

 by which it is preserved in this specimen, and will not expect to find 

 it soon again. 



The central region is occupied by five rounded or very obtusely 

 polygonal plates, interradially disposed, rather elliptic in outline. 

 The two antero-lateral plates are tolerably good-sized, and the 

 postero-lateral ones slightly smaller. All four of them have a con- 

 siderable thickness, extending downward below^ the level of the 

 ambulacra, and also rising somewhat above it. The posterior plate 

 is nearly three times as large as any of the others, and almost twice 

 as long as wide, extending well in between the two postero-lateral 

 plates. 



The relative positions of these plates are exactly like those of the 

 five plates at the summit of the forms of Platycrinus illustrated on 



