Il8 SMITHSONIAN INSTITUTION 



growth, in which all the plates occupy their original position forming essential 

 parts of the calyx wall ; the inf rabasals and basals, therefore, are more or less 

 erect and help to enclose the visceral cavity. 



A&. In Forbesiocrinus (text-fig. 10) the basals have become more oblique, 

 and the infrabasals, now recumbent, have decreased in proportional size so 

 that they form only a small part of the calyx wall, being chiefly a support for 

 the overlying plates. 



These two sections range from the Silurian to the Lower Carboniferous, 

 culminating with Forbesiocrinus in the Warsaw. 



Ac. In Ichthyocrinus (text-fig. 1 1 ) the basals approach very nearly the 

 horizontal, while the infrabasals have entirely withdrawn from the exterior, 

 taking no part whatever in the calyx wall. 



If we consider infrabasals instead of basals, it will be seen that section Aa 

 is comparable among Recent forms to Ptilocrinus and allied genera; Ab to the 

 Pentacrinidae ; Ac to the Comatulids ; and these would make three well-balanced 

 families if the distribution of the several phases of this character among them 

 were well proportioned and constant. In fact, however, while there is a very 

 sharp distinction among the genera between section c on the one hand and a 

 plus b on the other, this is not the case as between the last two, although the 

 typical examples of them are well differentiated; they shade into one another 

 to a considerable extent, and their separation, which is very desirable from a 

 taxonomic point of view, must be reinforced as far as possible by other charac- 

 ters. These sections, therefore, will form the three families of this group, viz. : 

 Aa, Lecanocrinidae ; Ab, Sagenocrinidae ; Ac, Ichthyocrinidae. 



The Lecanocrinidae, representing the most primitive stage of infrabasal 

 development, in which those plates are more or less erect, are also characterized 

 generally by a rather short, rotund crown. The Sagenocrinidae, in the typical 

 forms of which (e. g., Forbesiocrinus rather than the genus from which the 

 name is taken) the infrabasals have become to a large extent recumbent and 

 take little part in the cup enclosing the visceral mass, have usually an elongate, 

 spreading crown. Certain genera like Cholocrinus and Clidochirus cannot be 

 very satisfactorily placed on account of the intermingling of characters. As 

 between these two sections and the Ichthyocrinidae there is not much over- 

 lapping in the condition of the infrabasals. Occasionally in section b these 

 plates may not quite appear at the exterior, and thus would not be visible if the 

 column were attached; but in such cases it will be found that there is at the 

 lower part of the plates a flange-like projection by which they extend outward 

 beyond the proximal pentagonal space within the ring of basals (Pis. XXVI, 

 figs. 2a, b; XXVIII, fig. 19; XXXI, figs. 4a, b; XLIII, fig. 10). This is well 

 shown by many examples, especially in Forbesiocrinus where the infrabasals 



