KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 25. N:0 2. 41 



were probably corapound; but this may not have been the case in every speeies, or 

 indeed in every individual of H. Fléteheri. Variation in this point ocemrs in other Hete- 

 rocrinidae, notabh' in Anomalocrinus. This character then can hardly be said to ditfe- 

 rentiate the genus from Heterocrinus, 



The Arms, which are only known in the type-species, though supernciaUy like 

 those of Homocrinus, are slightly more specialised. The arms biftircate on about the (itli 

 or 7th primibrach, and from the inner sides of each branch armlets are given off at 

 intervals of ä or 6 ossicles. These armlets are halt the width of the mak) branch, extend 

 to a considerable lengtli, perhaps to the ends of the arms. and do not as a rule appear 

 ti» branch again. Such a mode of branching is easilv derived from a dichotomous arm of 

 the type of Heterocrinus escilis, 1 ) but I am not acquainted with anv other instanees of it. 

 Bv itself it would hardly be enough to differentiate the genus from Heterocrinus; and, 

 indeed, considering the variation of arm-structure in other Heterocrinidse, this particular 

 type may be confined to some specimens of H. Fletcheri. 



The Ventral Tube is mentioned by Angelin, and is represented in his Tal). X 

 tig. 24. It verv closelv resembles an arm, and the ossicles that form the main series 

 of the tube can hardly be distinguished from arm-ossicles; they have, like them, a 

 ventral groove, which is eovered with small plates, and they differ only in the fact that 

 their ventral sides are irregnlarly notched for the reception of these eovering-plates, as 

 in Calceocrinus pinnulatus. The tube in specimen /: lias a very faint median ridge, on 

 either side of which, at the snture, is a slight depression. The ventral tube of Hetero- 

 crinus appears, so far as it is known, to be of similar structure; but in that genus it 

 has not sunk so far down between the rays as in Herpetocrinus (see Pl. I, tigs. 1") — 28, 

 37, 38 and 49). 



The Steni forms the one great diagnostie teature of this genus. It is a remarkable 

 example of modification, and it is dittieult to distinguish the characters that are the result 

 of modification from those that are primitive. This difhculty is increased by the obscurity 

 that still overshadows the origin of the Pelmatozoan etern. 



It is convenient in describing the stem to divide it into regions. The regions that 

 can be dististinguished in the type-species may l>e uumbered, and will thus serve as a 

 standard by which to compare öther speeies. They are as follows (see Pl. I, tigs. 29, 36), 

 (1) Proximal: evolute; seetion circular; no ossicles beair ein-i. 



elliptic; 



» ; » » » 



semilunar; few » 



subcrescentic; several ossicles bear eirri. 

 subquadrangular; all ossicles bear eirri. 



These regions grade into one another; the shortest are 1, 2, and 3; while 5 and 6 

 form the main part of the stem and are those that best display the distinctive features 

 of the genus. 



(2) 



: » ; 



» 



(3) 



: in vol ute; 



» 



(4) 



: » ; 



» 



(5) 



: » ; 



» 



(6) Distal: 



» ; 



» 



') Brit. Foss. Crin., Ann. Ma^. Nat. Hist., ser. 6, Vol. V, Pl. XV, fig. 5 a. 

 K. Sv. Vel Aka.l IlauJI Band 25. N:o •-'. 



