92. PROCEEDINGS OF THE ACADEMY OF [1887. 



crinus, Stephanocrinus or Granatocrimis. In the three latter types^ 

 the so-called covering does not extend to the spiracles, but consists 

 apparently of a sort of moveable plates, by means of which in va- 

 rious ways the anal aperture could temporarily be opened or closed 

 as its functions required. 



The views exjjressed by Etheridge and Carpenter that these spine- 

 like pieces forming the pyramid are nothing but the proximal pin- 

 nules, cannot, in our opinion, be sustained by any of the evidence. 

 There are very serious objections to it : 



1. They consist of a single piece throughout their entire length, 

 whereas pinnules are composed of small joints. The specimens all 

 show this distinction well, and it may be clearly seen in Eth- 

 eridge and Carpenter's PI. V, fig. 28. 



2. They have no ventral groove, and taper to a point ; while 

 pinnules are nearly uniform throughout, and esj^ecially do not 

 taper perceptibly from their bases. 



3. They are more robust than the pinnules in the same specimen, 

 and shorter the pinnules jiassing beyond their tips. 



4. The best preserved specimens show that the pinnule sockets end 

 at the spiracles where the two rows of adjoining ambulacra come to- 

 gether in a jioint. The spines, however, seem to begin where the 

 pinnules end, and extend from there inward, the oiusters widening 

 toward the center so as to form the retreating angles at the base 

 of the pyramid. 



5. The spines are interradial and inter ambulacral, and as such 

 may belong to an interambulacral system, which perhaps is unre- 

 presented in other groups of the Blastoids, but certainly form no 

 part of the ambulacral system. 



Whatever the spines in Pentremites may be, or represent mor- 

 phologically, we think it will have to be conceded that they are not 

 "proximal pinnules," and not comparable to the plates covering the 

 anus of Orophocrinus, Stephanocrinus or Granatocrimis. 



On page 73, Messrs. Etheridge and Car2:)enter attempt to establish 

 a series of variations in the summit j^lates of the Blastoids, "similar 

 to that which can be traced among the Palaeocrinoids. The sim- 

 plest form of summit which occurs in any Blastoid is that presented 

 by Stephanocrinus. The peristome is completely closed by the five 

 triangular plates of the so-called proboscis." They state that Hall, 

 in his diagram of the structure of the summit in Elaeacrinus elegans ^ 



1 15th. Rep. N. Y. St. Cab. Nat. Hist. 1862, p. 153. 



