L52 



Gissocrinus. 



(Plate VIII, fig. 261. — Pl. X, fig. 402). 



1878. Gissocrivus, Angelin. Iconographia, p. 1<>. 



1879. Gissocrinus, Wachsmitu and Springer, Revision I, <s!t (Proc. j ( . 312). 



Revised Generic Diagnosis. 



I BB variable in number, owing to fusion, more or less complete, of one or tura 

 pairs. BB 5, hexagonal, except post. B, which is heptagona] and supports x. RR 5, shield- 

 shaped, with facet circular or elliptic in outline and occupying from 1 / 3 to 2 / 3 width of R. 

 x tetragonal to hexagonal, in line with RR, and about 2 /s width of R. Arms long. sira])le. 

 dichotomising regularly several times; with alternating compound covering-plates. Yentral 

 sac long, eomposed of plates which are primitively hexagonal and alternating, but which 

 usually become more or less tetragonal and in line; always laterally folded. Madreporite 

 distinet. Columnals low, alternating. 



This is a difficult genus to diagnose. Although elearly understood by Angelin, it 

 does not hitherto seem to have been properly appreciated. It has indeed been regarded 

 as little else than a Cyathocrinus with 3 instead of 5 infrabasals. While, however, the 3 

 infrabasals are by no means invariable, there are niany features common to nearlv all 

 species of this genus, which are hardly at all developed in Cyathocrinus. Such are: the 

 elearly marked axial ridging (not exactly folding) of the cup-plates; the lateral comprrs- 

 sion of the distal regions of the arms; the elevated rim at the distal end of each brachial 

 in ma ny species, or the ridging of the brachials in other species; the strong lateral or 

 transverse folding of the plates of the ventral sac; the length of the ventral sac and itfi 

 anteroposterior compression. It is apparently to the cornice-like rims of the brachials that 

 the genus owes its nanie; these, however, are not always present; when the v are, the 

 backs of the brachials are usually rounded; but when they are absent, the backs of the 

 brachials are ridged. The tendency to ornament, as manifested in the rims of the brachi- 

 als and the axial ridging of the cup, is sometimes carried to excess, and varies verv 

 considerably in the different individuals of a species. 



The fusion of the infrabasals is not of that complete and highly developed type 

 that obtains in Platycrinus. There are, apparently, cases in which no fusion has taken place, 

 at all events in the young; other cases in which only one pair of infrabasals has fused. 

 making a total of four (p. 1(>1); and tinally cases in which two pairs have fused: but 

 there are no particular pairs that tend to fuse more than anv others. It is rarely the 

 case that the unfused infrabasal is of larger size than a tifth of the circlet; and traces of 

 the original sutures are often to be distinguished between fused infrabasals. No morpho- 

 logieal importance can therefore be attached to the fusion, nor can theories of orientation 

 be based on it. 



