CRINOIDEA. 61 



Megistocrinus and Stereocrinus 

 E. B. Branson H. E. Wilson 



Branson wrote the descriptions of the new species of Megistocrinus and Stereo- 

 crinus in 1918. In February 1922 he sent all of the materials and manuscript to 

 Wilson. Wilson revised all specific descriptions and wrote the generic descriptions 

 All morphological discussion in the footnotes should be credited entirely to Wilson. 



All of the drawings of plates 6, 7, and 8, were made by Mr. G. T. Kline under 

 the directions of Branson. 



Genus Stereocrinus Barris 



Calyx an oblate spheroid; convex or flattened orally; convex, flattened or de- 

 pressed aborally: Height to width as 1:1 to 1:2.5. Tegmen 1/3 to 1/5 height of calyx. 

 Basal cycle discoidal; surface convex, flattened, or depressed to funnel-shape, nearly 

 concealed by columnals; exposed margin narrow; axial canal large, quinquelobate, 

 from 1/4 to 1/2 the diameter of the basal cycle. Basals (B) 3, subequal; 2 compound, 

 reduced laterally, 1 simple, laterally enlarged, basal formula usually ab-c-de-, with com- 

 plete ankylosis the formula becomes abcde. Radials (R), in contact laterally, subequal; 

 2 supported by compound basals only, hexagonal, 3 supported by two basals each 

 heptagonal; all hexagonal when all basals are ankylosed 1 forming a basal disk. Primi- 

 brachs (I Br) separated laterally, about equal in area to radials but higher and nar- 

 rower; pentagonal, axillary, and homologous to I Ax only. 2 Secundibrachs (II Br) 

 all pinnulate; II Bri pentagonal with subtriangular outline supporting II Br2 on the 

 outer side of the dichotome and one pinnule on the inner. II Br2 subtriangular; II Br3 

 et seq., biserially arranged and incorporated to at least II Bre in larger specimens; 

 the number of incorporated brachials varying with the size and age of the individual. 

 Pinnules 3 (P) of lower II Br partially incorporated, those above, free; incorporated 

 pinnules (II tiP) free above second pinnular (II nPr2); upper half of incorporated pin- 

 nulars II nPir2 to II nP 4 r 2 inflexed forming a portion of the tegmenal margin; neuro- 

 ambulacral canals of II nP present in immature specimens but absent in many larger, 

 mature forms showing that the free portion of the pinnule was lost and the canal per- 

 manently closed, during ontogenetic development. Interbrachials (iBr) 2, 3, or 4, 

 uniserially arranged; iBn the largest plate in the calyx; iBr 2 et seq., consecutively re- 

 duced in area. Incorporated interpinnulars inP rare; present only in large specimens. 



Arms bifurcating but once, biserial and pinnulate; size and length unknown; 

 arm bases horizontally directed; enlarging by spreading rather than by growth, thus 

 bringing more brachials and their pinnules into the cup wall. 



>The term basal disk has been loosely applied to any group of ankylosed, or unankylosed 

 basals having discoidal proportions and as this usage tends to confusion I believe the term 

 should be applied only where complete ankylosis has taken place. 



•I Ax in all forms having but one primibrach has usually been considered a compound 

 plate but the tendencies in Dolatocrinus, from which Stereocrinus originated, show clearly that 

 elimination by suppression and not ankylosis caused the loss of the I Br. 



•Pinnules, especially incorporated pinnules, form an important as well as a conspicuous 

 portion of the skeleton in many genera of crinoids and the following terms and symbols are 

 suggested for their technical description: Pinnules P; on secundibrachs II P, on tertibrachs 

 III P, etc. Pinnules incorporated, nP; on secundibrachs II nP, etc. Pinnulars Pr; on pinnules 

 of secundibrachs II Pr, etc. Pinnulars incorporated nPr. 



Interpinnulars iPr. 



Interpinnulars incorporated inPr. 



Following Dr. Bather's system of symbolism a step further we may designate any in- 

 corporated pinnular in the following manner: 



ex. II nPir« meaning the second, incorporated pinnular of the pinnule arising from the first 

 secundibrach. 



