436 ME. p. A. BATHEK ON PETALOCKINUS. [Aug. 1898, 



and greater flatness of the fan as a whole. So marked are these 

 features that they alone would suffice to distinguish the species. They 

 are gerontic characters, as already explained under P. vishycensis 

 (senior), hut in this case they affect the whole fan to a far greater 

 extent, and must be considered as phylogerontic, a view which 

 consists thoroughly with the geological horizon of the specimen. 



YI. The Affinities of Petalocrinus, 



The arm-fan of Petalocrinus, considered by itself, presents a 

 problem that the most accomplished specialist could hardly be ex- 

 pected to solve offhand. The solution here proposed was reached 

 only with labour and patience ; and even when confirmed by Mrs. 

 Davidson's fortunate discovery of a complete crown, there were 

 many details still obscure to those who had only the American 

 specimens for study. 



That the arm-fan has arisen by fusion of the branches of a dicho- 

 tomous, non-pinnulate arm, is proved by the evidence given in 

 detail above, namely : the law of branching of the grooves, the same 

 as the law of arm-branching in a Cyathocrinoid ; the emphasis of 

 the dorsal depressions that correspond to the ventral ridges as they 

 near the distal edge ; the undulating outline of the distal edge, 

 connected with the concavity of the ventral surface of the ridges, 

 especially in the distal region ; the lobation of the fan in corre- 

 spondence with the dichotoms ; the fine furrows sometimes seen on 

 the ventral surface of the ridges, pigmented in P. angustus, and 

 appearing in many sections, both horizontal and transverse, as fine 

 black lines indicative of pre-existing soft tissue ; the greater definite- 

 ness of this line along the median ridge, that is, between the two 

 main rami or half-arms ; the axial canals passing under the grooves, 

 branching with them, and often merging into them at the distal 

 end ; the notches for covering-plates, if not traces of the actual 

 plates. In the arm-fan as it at present exists, however, the original 

 branches are very thoroughly fused, not merely b}^ the lateral union 

 of the brachials, accompanied by their fusion in a vertical direc- 

 tion, but also by the deposit of secondary stereom. The closeness 

 of the ultimate connexion appears from the facts that fracture of 

 the arm-fan always takes place along planes at a definite angle to 

 the sagittal plane, and that the cleavage-planes cut cleanly across 

 all the ridges and grooves. The whole arm-fan, therefore, con- 

 stitutes, like any single ossicle of an echinoderm, a crystallographic 

 unit. 



Dichotomous arms following the regular law of arm-branching 

 are more common among Dicyclica than Monocyclica ; but to which 

 of these Sub-Classes Petalocrinus belongs must be decided by the 

 structure of the cup. This, it is claimed, shows a dicyclic base. 



The simple structure of the cup, and the relation of the first 

 primibrachs to the radials, proves that Petalocrinus belongs to the 

 Order Dicyclica Inadunata. 



The solidity of the tegmen, and the apparent prominence of five 



