. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 142 Mr. E. Billings on the Structure of XVI.—Notes on the Structure of the Crinoidea, Cystidea, and Blastoidea. By E. Billings, , Palasontologist of the Geological Survey of Canada*. [Continued from vol. v. p. 416.] 6. On some points relating to the Structure o/Pentremites. Professor Wyville 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 ^'periso7
. The Annals and magazine of natural history; zoology, botany, and geology. Natural history; Zoology; Botany; Geology. 142 Mr. E. Billings on the Structure of XVI.—Notes on the Structure of the Crinoidea, Cystidea, and Blastoidea. By E. Billings, , Palasontologist of the Geological Survey of Canada*. [Continued from vol. v. p. 416.] 6. On some points relating to the Structure o/Pentremites. Professor Wyville 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 ^'periso7natic'' systems f. These he considers to be Fiff. 1. Fig. 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, &; Philosophical Transactions of the Royal Society, vo
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