THE CRINOIDEA 



above IIBr (except in Thylacocrimis) ; 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. Archaeocrinus, 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 



IVBr 



IRBr 



IllBr. 



Fio. CXXV. 



Archaeocrimis desideratm, from the left 

 posterior radius. (Diagrammatised from 

 Waclismuth & Springer.) x ^. 



Fio. CXXVI. 



Thylacocrimis Vannioti (from Brit. 

 Mus. E6642). Seen from posterior inter- 

 radius. Owing to slight crushing all 

 five infrabasals (IB) are seen, and the 

 absence of a stem-facet may be noted. 

 Supplementary plates are shaded. The 

 section of an arm with pinnules (pn) is 

 after Oehlert. x 2. 



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. CXXVI.), has arms fixed up to I IIBr and sometimes 

 IVBr, after which follow long uiibranched biserial rami ; supplementary 

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

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

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



