Uemarks on the Genxs Pterotocrinus. 7 



thickened aud rounded above and slightl}^ thinner below; in some they 

 terminate in thin, rounded edges; in others they are tapered down to 

 an awl-like point, while yet others are slightly or widely bifurcate at 

 the extremities. They are so articulated to the angles of the vault, 

 that their inferior process divides the arms into groups of four, while 

 their single or bifurcate extremities extend outside of the pelvis, 

 having in some species a length bejoud it equal to its greatest 

 diameter. The lower process of these rays sometimes separates the 

 arms doAvn to the brachials, and sometimes onlj' through the upper 

 half of their extent. It is beyond conjecture what purpose in the 

 ph^ siological economy of the animal could have been served by them, 

 as they wei-e not needed for arm supports, aud their position shows us 

 plainly that they formed no part of the alimentary or reproductive 

 systems. They are entirely homologous in position, in the fact of their 

 articulation to the vault, and in separating the arms into equal groups, 

 with the so called axillaries of Eucalijptocrums* If the articulation 

 to the lower plates in the angles of the vault shall prove to be in any 

 wa}^ connected with furthur openings of the latter, their use ma^^ be 

 traced to some of its functions; but this at present we have no means 

 of determining. 'J^ie articulation seems, in some cases, to have been 

 by means of separate pieces rather than b}' processes. 



The Column. 

 The small part of the upper portion preserved is very slender, round 

 and composed of equal, very thin pieces. As but slight remains of it 

 have been found, our knowledge is restricted to the above statements. 



Remarks. 



It will be seen from the description of this genus, and of the species 

 placed in it by its authors, that the Dichocrinus cornigerus and Jjicho- 

 crinus sexiobatus of Shuraard can not be referred to it, as has been er- 



* From a study of fine specimens of E. crassun, kindly loaned me by Dr. R. M. Byrnes, 

 two of which are figured on the plate accompanying this article, it is evident that the inter- 

 brachial rays of Pterotocrinux are articulated to the vault in much the same manner as the 

 axillary and interradial raj-s of Eucahjptocrinus. In the latter genus, the base of the 

 vault is exeava^f d at its angles, for the reception of the lower inner protuberance of these 

 rays ; these excavations are larger for the interradial than for the axillary rays. As they 

 alternate, the arms are divided into groups of fours between each pair of axillaries or inter- 

 radials. The ambulacral grooves are continued into the pelvic cavity through the openings 

 in the basal pieces of the vault. The latter narrows upward, and opens at the apex by a 

 small, central aperture, closed by five minute, pyramidal plates.. Good casts of this spe- 

 cies, from Cedarville, Ohio, now in my cabinet, show the arrangement of the rays and the 

 openings of the pores in the bases of the arms most satisfactorily. One of these is figured 

 with the specimens from Dr. B's collection. This figure will be readily understood by re- 

 ferring the elevations on the cast to depressions in the Crinoid, and viceverm. A full study 

 of this genus, from numerous cut specimens, is in course of preparation, and will be pub- 

 lished soon . 



