356 PROCEEDINGS OF THE ACADEMY OP [1881. 



fereiiccs, even if they were persistent, can be deemed sufficient for 

 a family distinction. Thysanocrinus of the Rhodocrinidje has 

 generally at four sides the first interradial disposed between the first 

 and second radials ; while in Thijlacocrinus, according to Oehlert's 

 figure, all five first interradials rest directly upon the basals, or to 

 use Zittel's language, "form a ring of ten plates with the first 

 radials as in his Rhodocrinidse.^^ 



In our classification, we place under the Rhodocrinidje all genera 

 of the Spha^roidocrinidfe which have well-defined underbasals, and 

 we admit also Glyptoerinus in which these plates are exceedingly 

 rudimentary, or perhaps in some species altogether undeveloped. 

 The Rhodocrinidae have five — exceptionally three — basals ; from 

 2 to 3X5 primary radials, 2 to 6 X 10 secondary and some- 

 times several tertiary radials. The pjates of the higher orders, if 

 such are present, being formed into extended free ra3^s with 

 lateral arms. Arms rather delicate, branching or simple, the latter 

 being rather the exception. Interradials numerous, arranged with 

 two or three plates in each series, except in the first which has 

 but one plate. In most of the genera, the first interradial rests 

 directly upon the truncate upper side of the basals, thereby 

 separating the first radials all around. In others, however, only the 

 first plate of the posterior or anal side is supported by a basal, 

 that of the other four sides being placed against the upper corners 

 of the first and between the second radials, the former producing 

 an almost perfect pentrahedral, the latter a more or less bilateral 

 symmetry. 



In Glyptoerinus, the first interradial rests between the first and 

 second radials at every side ; interaxillaries generally present ; 

 vault fiat or low hemispherical, composed of small and more or 

 less irregular pieces ; apical dome-plates not well defined ; inter- 

 radial regions depressed ; anus in form of a simple opening 

 through the vault, rarely proboscis-like. Column round or pen- 

 tagonal. 



For greater convenience of study we subdivide the Rhodo- 

 crinidre into three groups. 



a. Glyptocrinites. — Calyx turbinate, symmetry almost perfectly 

 pentahedral, radial plates with rounded, strongly elevated ridges 

 which gently pass into the arms. Interradial areas depressed, the 

 first plate resting either directly upon the basals, or between the 

 second and third radials, without special anal plate beneath their 

 line. Restricted to the Lower Silurian. 



