KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND 25. \:<> 2. 169 



is hardly a step; the latter species, however, has rione of the characters that differentiate 

 Arachnocrinus. On the other hand, G. arthriticus Phillips, which only escaped being 

 made the type of Gissocrihus owing to its absenee from Gotland, has almost all the 

 characters of Arachnocrinus. Really the only difference between Arachnocrinus and some 

 species of Gissocrinus lies in the fäet that the former has 5, the latter 3 infrabasals. But 

 it must be remembered on the one hand that many specimens of Gissocrinus do actually 

 show traces of the original pentamerous symmetry in the base, and on the other hand 

 that the infrabasals of the American species of Arachnocrinus are quite minute and have 

 been properly described for no species. I incline therefore to the opinion that the American 

 species may eventually be found to possess three infrabasals only, in which case the two 

 genera could no longer be separated. 



For the present, as I have not vet had the opportunity of examiriing the American 

 specimens, I retain the genus and the name Gissocrinus. ]f, however, my suspicions are 

 well founded, it will some day be necessary to consider whether the genus should not be 

 called Arachnocrinus, and A. bulbosus Hall sp. taken as its type. Such a proceeding 

 would certainlv be a charming reductio ad absurdum of one custom of nomenclaturists, 

 for the diagnosis of Arachnocrinus would no longer retain any of its original clauses. 



Description. 



Dorsal Cup very low and proportionaliv wide. Its plates are simply curved, but 

 not swollen, except that the radials project slightlv; thev appear to have been ornamented 

 with small tubercles, espeeially along the margins, but it is not certain that this was the 

 case in all specimens. The measurements of a small specimen are: height 5 mm., widtli 

 below estimated at 3.5 mm., width above estimated at 8.5 mm. (figs. 335, 337, 341, 342). 



IBB number not ascertained; low, minute. 



BB 5: hexagonal, except post. B which is heptagonal. The height is at most 2 / 3 

 the widtli. 



RR 5: slightlv wider than high. The facet occupies s / 4 of the width and is almost 

 vertical. The axial canal is distinct as a rule, but placed rather close to the ventral 

 groove, with which it is sometimes connected (fig. 338). 



The Arms extend horizontally from the catyx to the level of IIIBr or IVBr; here 

 they begin to curve round, dichotomising as they go (figs. 335, 336). The distal branches 

 are remarkably involute, sometimes forming two distinct whorls; the involution also appears 

 to be constant (fig. 351). The ossicles are roughlv quadrangular in section, with a broad 

 rounded back and flat inbent sides, which suddenly curve in to a rather narrow ventral 

 groove. The axial canal can be traced for some distance, but lies very close beneath the 

 groove and joins it in the distal ossicles (figs. 344 — 346, 349, 350). 



The covering-plates were composed of at least the elements c 1 and c 2 and some 

 internal pieces (figs. 344, 345, 346, 349). The inner rows of plates (c 2 ) appear to have 

 been depressed; but so peculiar an appearance may be due to the state of preservation. 



IBr 1, possibly sometimes 2. Occasionally thev are very low and almost triangulär, 

 but usually are rather longer than wide. They are constricted in the middle and either 



K. Sv. Vet. Akad. Handl. Band. 25. N:o •,'. 22 



