THE FOSSIL CRINOID GENUS DOLATOCRINUS AND ITS ALLIES. 11 



20, and therefore with only three orders of brachials, are well shown. 

 The most remarkable thing about this form, however, is the condi- 

 tion of the base. With the radials included, there is a shallow basal 

 cavity in which the basals are actually wanting in the specimens, 

 but in one traces of the attachment of four small remnants of plates 

 are seen inside the ring of radials (text fig. 4). Although their 

 presence is indicated by crenulae in the small notches at the junction 

 of the radials, the basals were practically eliminated from the calyx. 

 They are entirely absent in the 

 three specimens showing these 

 parts, the proximal edge of the 

 radials being deeply incised by 

 the lobes of the axial canal. A 

 huge column facet enveloped the 

 entire base, including the radials 

 and part of the next range; and 

 the basals, thus shut out from 

 the exterior, tended to disappear 

 by atrophy. Such an almost 

 complete elimination of basals 

 has not been observed in any 

 other Camerate crinoid, but is 

 paralleled among the Flexibilia 

 by the Silurian genus Cltisto- 

 crinus Springer, Crinoidea Flexi- 

 bilia (pi. 38, fig. 2b), and as to infrabasals by several forms of the 

 Ichthyocrinidae. 



The radials and primibrachs are small, succeeded by large plates in 

 the brachial series, which are singularly irregular in form and size. 

 The half rays bifurate on the second secundibrach, which is followed 

 by about four ranges of tertibrachs and one or two biserial pairs of 

 arm brachials before the arms become free. The interbrachials are 

 in six or seven ranges, beginning with a large first plate followed b}' 

 two, with an extra plate interpolated on the posterior side. A nar- 

 row series, rarely exceeding two plates abreast, continues to the 

 zone of the arm bases, but the fourth range consists invariably of a 

 single plate. Similar plates are well developed in the second and 

 third axils. 



The cah'x plates are smooth, entirely devoid of the intense sculp- 

 turing seen in H. pZemssiwus; but they have the very unusual char- 

 acter of broad, shallow median depressions containing smaller pits, 

 which are double or triple on the radial series and single on the inter- 

 radial, the triple pits being on the axillaries. 

 183081—21 2 



Fig. 4.— Base in Hadrocrinus discus. Remnants 

 of atrophied basals indicated by notches; 



RADIALS INDENTED BY AXIAL CANAL AND C0\T:RED 

 BY COLUMN, AS SHOWN BY' THE RADIATING STRIAE. 



