1890.] NATURAL SCIENCES OE PIIILADELPHIA. 369 



In Glyptocrhms and 31egistocrintis, a sort of vault is formed by tlie 

 greater development of the interanibuhua-al plates and their gradual 

 fusion along the line of the ambulacra, and from that it would seem 

 to follow that similar modifications took place also in the later forms 

 if it was not for the su])posed disk at the inner floor of some species 

 of Batocrinus, Actinocrbius, Physetocrinns etc. As to this structure 

 we have recently made some very important observations. 



Through the kindness of Dr. Horace G. Griffith of Burlington, to 

 whom we are indebted for many favors, we obtained a most instruct- 

 ive specimen of Physetocrinus showing the structure along the inner 

 floor of the tegmen in a most excellent preservation. This spec- 

 imen has the great advantage of being free from any silicious coat- 

 ing, such as obscured the structure in all former specimens of 

 this kind. The outer surface of the tegmen is composed of mod- 

 erately large, smooth pieces of irregular form, closely fitted to- 

 gether at all sides. There are no orals, but near its outer margin 

 there are " radial dome plates " of a first and second order, which 

 are readily recognized by their larger size and greater convexity ; 

 but besides these no other covering pieces are visible. Looking at 

 the inner floor, we find the same arrangement of plates, and actually 

 the same plates, but their general aspect is totally different. They 

 appear as sharply delineated stars, with as many i-ays as there are 

 sides to the plates. There are abrupt depressions between the star- 

 rays, which on meeting the corresponding depressions of adjoining 

 plates, form deep, sometimes cavernous pits, more or less undermining 

 the plates ; and there seems to be but little doubt that the pits commu- 

 nicated with one another by imbedded passages all along the teg- 

 men. The star-shaped plates extend over the peristome as well as 

 over the ambulacra, but are occasionally interrupted by small, ir- 

 regular su pplementary pieces, apparently solid. The structure shows 

 plainly that the tegmen of Physetocrinus is not composed of two dis- 

 tinct sets of superimposed plates, but of one set only, which are solid 

 externally, and more or less perforated or porous at their inner por- 

 tions. That there is but one set of plates is further confirmed by the 

 position of the ambulacra which, as shown by the specimen, follow the 

 inner floor. The latter is of the utmost importance, for, if the upper 

 or solid part, as was supposed to be the case in the allied Batocrhnis 

 and Adinocrinus, represented a vault, and the inner part a disk, 

 the ambulacra, if placed beneath the latter, would be covered by 

 two integuments, at first by the overlapping interambulacral plates. 

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