1879.] NATURAL SCIENCES OF PHILADELPHIA. 241 



of the anal series as an element in the structure of many Paleo- 

 crinoids, may be largely clue to the solid dome, which has to be 

 penetrated by a special aperture, requiring some modification of 

 the general structure below to accommodate it. It seems to have 

 no direct representative in the apical system of the recent Echino- 

 derms, but we may be justified in considering it as a specialized 

 interradial, and in that case the basals of the forms under con- 

 sideration are found to conform entirely to Carpenter's interpre- 

 tation, being interradially disposed. We find a most interesting 

 confirmation of this view in a specimen of Actinocrinus (Sfroto- 

 crinus) umbrosus, which has abnormally no first anal plate, the 

 first radials joining at their sides. The anal series in form and 

 proportions is very similar. to the other interradials, being chiefly 

 distinguished by having three plates in the second range instead 

 of two, as in the others. Accordingly, we find the basal disc in 

 this specimen reduced to three unequal plates, and if we bisect 

 the two larger, we obtain five equal plates, interradially situated, 

 just as in Platycrinus. Nature herself, in this isolated specimen, 

 has thus beautifully illustrated our argument. It is well to note 

 in this connection that in Platycrinus, and all genera with three 

 unequal plates in the proximal ring, the small plate is never, so far 

 as we have observed, on the anal side, and this is the case with 

 the abnormal specimen above described, the small plate being 

 situated below the suture of the left posterior and lateral rays. 

 Why this is so we are as yet unable to explain. 



In forms like Gyathocrinas, Rhodocrinus, etc., which have two 

 rings of five plates each, the proximal plates are radially situated, 

 and, therefore, according to Carpenter, cannot be basals or homo- 

 logous to the genitals, but the second ring of plates or "subradials," 

 being interradial in position, are the true basals and the homolo- 

 gues of the genitals. If, now, we examine those types with two 

 rings below the radials, in which the proximal ring consists of less 

 than five plates, we shall find his idea still further confirmed. 



In the Ichthyocrinidse (except Calpiocrinus?) which have in 

 the proximal ring three unequal plates, they are so proportioned 

 and so situated that if we divide them by two additional sutures 

 into five about equal plates, these fine will be radially situated, and 

 exactly equivalent to the corresponding set of plates in Gyatho- 

 crinus. In Gissocrinus, one of Angelin's Upper Silurian genera 

 from Gotland, which is in every other respect a true Cyathocri- 



