142 



Mr. E. Billings on the Structure of 



XVI. — Notes on the Structure of the Crinoidea, Cystidea, 

 and Blastoidea. By E. Billings, F.G.S., Palasontologist 

 of the Geological Survey of Canada*. 



[Continued from vol. v. p. 416.] 

 6. On some points relating to the Structure o/Pentremites. 



Professor Wy ville Thomson has proposed a division of the 

 skeleton of the existing Crinoid Atitedon rosaceus into two 

 systems of plates, which he terms respectively the " radial 

 sind the ^' per iso7natic'' systems f. These he considers to be 



Fiff. 1. 



Fig. 2. 



Fig. 1. Calvcine plates of Pentronitvs: h, the basals; /, one of the five 

 forked plates ; f7, deltoid plate ; /, lancet-plate ; os, oral spiracle ; s, 

 spiracle. Fig. 2. Caryocystites testudinarius, Hisinger : b, basal plates ; 

 r, radials; m, mouth. 



thoroughly distinct from each other in their structure and mode 

 of growth. The radial system consists of the joints of the 

 stem, the centro-dorsal plate, the radial plates, the joints of the 

 arms, and also those of the pinnules. In the perisomatic sys- 

 tem he includes the basal and oral plates, the anal plate, the 

 interradial plates, and any other plates or spicula which may 

 be developed in the perisome of the cup or disk. This I think 

 a good arrangement, except in so far as it regards the stem, 

 which a])})ears to me to be always an appendage of the peri- 

 somatic rather than of the radial system. 



Throughout the whole range of the Crinoidea, the plates of 

 the radial and perisomatic systems are easily distinguished 

 from each other. In general the Cystidea have no radial 

 plates in their calyces, except, perhaps, in a small area around 



* From ' Silliman's American Journal of Science,' Sept. 1870. 



t " On the Embryogeny of Antedon rosaceus, Linck (Comatula rosacea 

 of Lamarck), by Professor Wyville Thomson, LL.D. &c." Philosophical 

 Transactions of the Royal Society, vol. civ. part 2. p. 540. 



