1886.] NATURAL SCIENCES OP PHILADELPHIA. 123 



azygous side, which must be modified. We asserted that the 

 upper end of the anal plate possessed a peculiar crenulated sur- 

 face, a structure which Wetherby had previously described, and 

 which we thought we had observed also in some of his specimens. 

 The fact that the so-called " crenulation " was visible, more or 

 less, in every specimen which Wetherby had sent us for com- 

 parison, led us to believe that it was an organic structure, prob- 

 ably of a similar nature as the pectinated rhombs of the Cystidea. 

 We are now satisfied that we had not sufficiently taken into con- 

 sideration the peculiar state of preservation of Wetherby's 

 specimens, which are not only highly silicified, but the surface 

 of the plates is badly weathered, and more or less destroyed. 

 Through the kindness and liberality of Mr. Walter Billings we are 

 now enabled to give a more satisfactory interpretation of these 

 parts. On receiving our paper he promptly informed us that in 

 the Canadian species of Eybocriniis the azygous plate was not 

 crenulated, but crowded by numerous, very small pieces, forming 

 a short pyramid. He accompanied his description with good 

 figures, and offered us his specimens for examination. In these 

 we found, resting upon the upper side of the anal plates, several 

 rows of minute subquadrangular pieces, about six to the width 

 of the plate, which together form a tumular protuberance without 

 any visible opening. There appear, however, minute pores along 

 the sutures, and it is evident that the protuberance represents a 

 ventral sac, reduced to its minimum size. The crenulated 

 appearance in Wetherby's specimens is evidently the result of 

 weathering, the apparent grooves representing the sutures 

 between the small plates. A similar short protuberance has been 

 observed in Eybocystites, and in all probability was present in the 

 two other genera. 



No new species of this genus have been described since our 

 first discussion. We found several specimens in the Trenton 

 limestone, near Knoxville, Tennessee, but none of them perfect 

 enougli for specific identification. 



HYBOCYSTITES AVetherby. 



(Revised by W. and Sp.) 



1880. Wetherby, Cincin. Joum. Nat. Hist. (July). 



1882. P. Herb. Carpenter, Quart. Joum. Geol. Soc. London, pp. 298-312. 



Hxjhocystites combines, in a remarkable degree, some of the 

 characters of Palaeocrinoidea, Cystidea and Blastoidea. Wetherby 



