44 F. A. BATIIER, CR1NOIDEA OF GOTLAND. 



manner; and that cirri were given off from the pentanieres, between the sutnres. A spe- 

 cimen of //. jlabellicirrus, in which a niore distal portion of the stem is preserved than 

 is usually the c;isc, shows that in this region there was aetually a third cirrus between 

 the two outer median sutures (Plate II, fig. H6). 



The explanation of the stem-structure to which we have been led does not, it is 

 trne, harmonise with the law stated by Messrs. Wachsmuth and Springer; 1 ) for, accord- 

 ing to that law, the cirri, the angles of the axial canal, the longitudinal sutures and the 

 sides of the stem should all have the same orientation, whether radial as in Dicyclica or 

 interradial as in Monocyclica. Waghsmuth and Springer »have found this rnle to be 

 without exception aniong palaeozoic crinoids ; but it has often enough been pointed out 

 that exceptions occur aniong Neozoie Crinoids. Thus, - in Pentatirinus the cirri and 

 sides of the stem are radial, but the angles of the axial canal are interradial; in Bathy- 

 crinus the angles of the axial canal, as befits a Monocyclic Crinoid, are interradial, bnt 

 the sides of the proximal ossicles are radial; in Rhizocrinus, another Monocyclic form, the 

 sides of the stem are quite properly interradial, but the angles of the canal are also in- 

 terradial. The rnle does no doubt hold good for most Palaeozoic Crinoids, and hitherto 

 has been confirmed b}' every fresh fact. 2 ) ( _)n the other hand a stem occurs in the Si- 

 lurian of Gotland in which the angles of both stem and axial canal and the longitudinal 

 sutures all have the same orientation. These diversities of plan may be capable of ex- 

 planation; actual changes of orientation can, it appears, take place, or secondary structures 

 can obscure the primitive arrangement. Antedon />ifi<la, Pennant, sp. (— A. rosacea, 

 Linck, sp.) is a notable example of this: here the first-formed cirri are radial, but the 

 se< ond-formed cirri are interradial; downward extensions of the axial canals are radial, 

 but the angles of the lumen of the centrodorsa] in the adult are interradial; the angles 

 of the centrodorsa] in the adult are interradial, but in the young they are radial, and yet 

 again Mr. Bury has shown that these radial angles are merely infrabasals fused to the 

 top stem-ossicle, and therefore the angles of this ossicle, so far as it had any, were pri— 

 mitively interradial. This form then starts in a truly dicyclic stage, agreeing with 

 Wachsmuth and Springer's rule, passes through a pseudö-monocyclic stage. with apparent 

 exceptions to the rule, and tinally reaehes a monocyclic stage, in which any exceptions to 

 the monocyclic character are of secondary naturc and valueless as evidence. Further 

 knowledge will no doubt solve the many problems conneeted with the Pelmatozoan stem, 

 but it is early yet to be stating general laws. A rule with exceptions is of small value 

 in science, until the exceptions by being explained help to prove the rule. 



To return to the stem of Herpetocrinus. There may often be seen on the outside 

 a faint groove, looking almost like an obscured suture, which runs parallel with the outer 

 lateral sutures, close to them and on the outside of them (Pl. I, tig. 30). The meaning 

 of these grooves is not obvious; they may be merely the impressions produced by the 



') Revision 111, (7); Proc. 188. r >, p. 229. Revision III, Ci!U); Proc. 1886, p. 218. »Discovery of 

 ventral surface of Taxocrinus &c.» Proc. Acad. Nat. Sci. Philadelphia 1888, p. 351. 



-) Brit. Foss. Crin. III, Ann. Mag. Nat. Hist., s«'r. 6, Vol. VI, p. 230; and V, op. vit., ser. 6, Vol. 

 VII, p. 408. 



