200 



THE CRINOIDEA 



above IIBr (except in Thylacocrinus) ; illBr may or may not be present. 



The plates of the tegmen are small and usually irregular. The family 



begins in the Ordovician, probably as an independent development from 



Reteocrinidae or similar forms, and runs parallel to the Dimerocrinidae 



through the Silurian, but, unlike them, persists to Carboniferous times. 



There is seen in it an increase in definiteness of iBr, and the origin of 



biserial arms, which are usually isotomous but exceptionally bear ramules. 



Genera Rhaphanocrinus, W. & Sp. (1885 ; syn. Coelocrinus, Salter), 



Ordovician, N. America, Gt. Britain, has uniserial arms and numerous iBr 



and illBr. Archaeocrimis, W. & Sp. 



(1881), Ordovician, N. America, has 



Br in zigzag, numerous iBr more 



regular in arrangement, with no verti- 



cal series in anal IR (Fig. CXXV.). 



Diabolocrinus, W. & Sp. (1897), Ordo- 



vician, N. America, has the larger 



and more regular iBr surrounded by 



supplementary plates ; the anus is 



at the end of a strong, subcentral tube ; 



the arms bear ramules. Lyriocrinus, 



Hall (1852 ; syn. Marsupiocrinus, Hall 



non de Blainv. nee Phill.), Silurian, N. 



America and England, has but 2 rami 



IllBr 



Fio. CXXVI. 



Thylucocrimis yannioti (from Brit. 

 Mus. E6642). Seen from posterior inter- 

 radius. Owing to slight crushing all 

 live iufrabasals (IB) are seen, and the 

 absence of a stein-facet may be noted. 

 Supplementary plates are shaded. The 

 section of an arm with pinnules (pn) is 

 after Oehlert. x i 



IllBr 



Kio. CXXV. 



Archacocrinus desiderntuf, from the left 

 posterior radius. (Diagrammatised from 

 Wachsmuth & Springer.) x A. 



to each ray, biserial; iBr, 1 + 2 + 1, not always quite separating RR ; 

 anal area usually of similar structure. Anthemocrinus, W. & Sp. (1881). 

 Silurian, Gotland, has only one IBr, and the first large iBr is followed by 

 only 1 or 2 small ones ; the biserial arms fork 2 or 3 times. Diameno- 

 crinus, Oehlert (1891, em. Jaekel, 1895), Devonian, France and Germany, 

 has repeatedly isotomous arms, with Br in zigzag ; 6-8 IIBr, and a long 

 narrow series of iBr. Thylacocrinus, Oehlert (1878), Devonian, France 

 and New York (Fig. CXXVL), has arms fixed up to IIIBr and sometimes 

 IVBr, after which follow long unbranched biserial rami ; supplementary 

 plates occur between all fixed brachial series ; stem minute or (?) absent. 

 Lahiiseniocrinus, Tschernyschew (1892,-93), Lower Devonian, Ural, 

 appears allied to the preceding, but base and free orachials are unknown. 



