CLASS II , BLASTOIDEA 161 



Although a few forms {Echinosphaerifes, Aristocystites, Caryocystites) appear 

 in considerable abundance in certain formations, and are locally profuse in 

 some beds, the majority are of comparatively rare occurrence. The brachioles 

 are only exceptionally preserved, owing to their fragile constitution, and the 

 stem is also usually lost. 



Cystideans are found most plentifully in the Ordovician rocks of St. 

 Petersburg, Russia, and in the Silurian localities of Oeland, Gotland, Sweden, 

 Wales and Bohemia (Etage D). The Bohemian specimens are usually pre- 

 served in the form of casts and moulds, and are contained in siliceous or 

 argillaceous slates. The Chazy and Trenton limestones of Canada, New York, 

 Ohio and Indiana also yield a large variety of forms. 



Excellently preserved specimens of Pseudocrinites, Apiocystifes, Echinoen- 

 crinus and Anovialocystites are obtained from the Silurian limestones of Dudley 

 and Tividale, England ; and similar forms (Lepadocrinus, Callocystites, Caryo- 

 crinus) are found in the Silurian (Niagara Group) of North America. Only 

 scanty remains are known from the Devonian, and from the Lower Carboni- 

 ferous but a single genus, Lejndodiscus. 



Two genera have been described from the Permian, Hypocrinus Beyrich, 

 and Ooenocystis Girty ; but their systematic position is doubtful, and until more 

 is known of their structure they may be left out of consideration. 



Class 2. BLASTOIDEA Say.i 



Extinct, short-stemmed, or stemless Pelmatozoa with a rigid calyx resembling a 

 floioer-hud in shape, with pentamerous symmetry predominant {occasionally modified 

 by atrophy), usually composed of thirteen principal plates. Food-grooves lying in 

 lanceolate or linear areas {ambidacra or pseudamlndacra) which radiate from a 

 central peristome between fire interradiol deltoid plates and are not crossed by sutures 

 between calyx plates ; they bear at their lateral margins pinnule-like appendages, and 

 from their inner floor hang lamellar tubes known as hydrospires. Grooves and peri- 

 stome protected by small, movable covering plates. 



The ccdyx is clavate, pyriform, ovate or globose, frequently pentangular 

 at its upper face, and composed of plates which are firmly united among 

 themselves. The plates of the abactinal system are arranged in three suc- 

 cessive cycles, represented by the basals, radials and interradials or deltoids. 

 The plates of the actinal system comprise the summit plates and the ambulacra. 



The basals consist of two plates of equal size, and a third smaller one, 

 which is directed invariably toward the right anterior interradius. Eesting 

 upon the basals are five V-shaped, usually equal radials (commonly known 



1 Literature: ,Srt?/, T'., Observations on some Species of Zooiiliytes, etc. Anier. Joiirii. Sci. , 1820, 

 vol. ii. — Say, T., On two Genera and several Species of Crinoids. Journ. Acail. Nat. Sci. Pliilad., 

 1825, vol. iv. (Also in Zool. Journ., 1825, vol. ii.) — Roemer, F., Monographic der fossilen 

 Crinoidenfamilie der Blastoideen. Troschel's Archiv fiir Naturgescli. , 1851, Jahrg. xvii., vol. i. — 

 Rofe, J., Notes on Ecliinodermata. Geo!. Mag., Dec. 1, 1865, vol. ii.— Billings, E., Notes on the 

 Structure of Crinoidea, Cystoidea, and Blastoidea. Amer. Journ. Sci. 2nd ser., 1869-70, vols, 

 xlviii.-l. — Etheridge, R., and Carpenter, P. II., Catalogue of the Blastoidea in the Geological 

 Department of the British Museum, 1886. [Complete bihliogi-aphy, pp. 303-310.] — Bather, F. A., 

 Genera and Species of Blastoidea, with a list of specimens in the British Museum, 1899. [Complete 

 index of names with references to literature.] — Batlier, F. A., Treatise on Zoology (Lankester). 

 Part III., Echinoderma, 1900. — Hambach, G., Revision of the Blastoidea. Trans. Acad. Sci. 

 St. Louis, 1903, vol. xiii. — Hudson, G. H., Pelmatozoa from Chazy Limestone. New York State 

 Museum Bull. No. 107, 1907. 



VOL. I M 



