CLAss II BLASTOIDEA 161 



Although a fcvv forms (EchinosphaeriteSj Aristocystites, Caryocystites) appear 

 in considerable abuudaiice 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 (fitage 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 Fseudocrinites, Apiocystites, Echinoen-^ 

 crinus and Anomalocystites 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, Lepidodiscus. 



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

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

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



Olass 2. BLASTOIDEA Say.i 



Extind, short-stemmed, or stemless Pelmatozoa with a rigid calyx resemhling a 

 flower-bud in shape, witK pentamerous symmetry predominant (occa^ionally modified 

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

 lanceolate or linear areas {amhulacra or pseudambulacra) which radiale from a 

 central perisfome hetween five interradial deltoid plates and are not crossed hy sutures 

 hetween calyx plates ; they hear at their lateral margins pinnule-lihe appendages, and 

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

 stome protected by small, movable covering 



The calyx 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. Resting 

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



^ Literatiire : Say, T., Ohservations on some Species of Zoophytes, etc. Amei\ Journ. Sei., 1820, 

 vol. ii. — Say, T., On two Genera and several Species of Crinoids. Journ. Acad. Nat. Sei. Philad., 

 1825, vol. iv. (Also in Zool. Journ., 1825, vol. ii.) — Roemei\ F., Monographie der fossilen 

 Crinoidenfamilie der Blastoideen. Troschel's Archiv für Naturgesch. , 1851, Jahrg. xvii., vol. i. — 

 Jiofe, J., Notes on Echinodermata. Geol. Mag., Dec. 1, 1865, vol. ii. — Billings, K, Notes on the 

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

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

 Department of the British Museum, 1886. [Complete bibliogi-aphy, pp. 303-310.]— ßo^/jgr, 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.] — Bather, F. A., Treatise on Zoology (Lankester). 

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

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

 Museum Bull. No. 107, 1907. 



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