406 Mr. F. A. Bather on British Fossil Crinoids : 



pronounced axial folding of the cup, in the longer and 

 straighter arms, in the greater parallelism of the sutures 

 between the arm-ossicles, in the greater length and fineness 

 of the pinnules, in the less ridged nature of the ventral sac, 

 and apparently in the more pronouncedly pentagonal stem. 



General Eemarks on the Genus. 



In the investigation of the various species of this genus at 

 home and abroad certain points of morphological interest have 

 turned up, to which attention may now be directed. 



The covering-plates of the arms. — It has already been 

 remarked (p. 393) that the tegminal plates appear continuous 

 with the covering-plates of the arm, and this is partly seen in 

 PI. XIII. fig. 8. The specimen there shown would, how- 

 ever, not prove much by itself ; it is suggestive rather than 

 convincing : but in a large specimen of B. ramosissimus in 

 the Eiksmuseum at Stockholm the ventral surface of the arm 

 is completely exposed, and here is seen a remarkable structure 

 (fig. 4, p. 408) . The ventral groove, instead of being covered 

 by a double row of regularly alternating shield-shaped plates, is 

 entirely overlaid by a large number of small irregular plates 

 which appear to extend beyond the limits of the groove itself 

 over the adjacent portions of the arm-ossicles. Ordinary 

 covering-plates open along the median line, and that this was 

 the case in Boiryocrinus is proved by many specimens; but 

 in the present specimen there is neither visible opening nor 

 suggestion of opening, at least in the proximal part of the 

 arms. The appearance, though on a larger scale, is not 

 unlike that of the small plates that cover the ventral surface 

 of a pinnule swollen with genital products ; since, however, 

 the structure occurs not in the pinnules or armlets but in the 

 main arm it is improbable that the similarity is more than 

 superficial. On the other hand, the apparently swollen state 

 of the lower pinnule in PI. XIII. fig. 9 may be due to the 

 presence of gonads. 



The Ventral Sac. — I have previously alluded to the opinion 

 that " slit-like fissures " are present in the ventral sac of 

 certain Fistulata (Paper II., Annals, ser. 6, v. p. 311) ; and 

 I have shown that in Thenar vcrinus slits do not exist, although 

 the appearance of such is produced by simple folding of the 

 plates (Paper III., Annals, ser. 6, vi. p. 229). Now in this 

 connexion the genus Botryocrinns (including Sici/ocrinus) is 

 of particular interest. For, in the first place, the slit-like 

 appearance is, in many specimens of all species, very strongly 

 marked ; and, in the second place, a description of the ventral 



