390 PROCEEDINGS OF THE ACADEMY OF [1881. 



the British Association relative to descriptions by the earlier 

 writers, this entitles the name 0Z^ac7'tnus to priority, which leaves 

 Gilbertsocrinus, Goniaateroidocrinus and Trematocrinus as 

 synonyms. 



The nearest affinities of this genus are with Hhodocrinus, with 

 which it substantially agrees in the structure of the body below 

 the arms, but from which it is conspicuously distinguished by 

 the pseudo-brachial appendages, as well as the size and disposi- 

 tion of the true arms. The two genera are, however, connected 

 through the transition form 0. sjnnigerus Hall, which has the 

 arms and body similar to many species of Rhodocrinus^ yet the 

 pseudo-brachial appendages of Ollacrinus. 



These appendages are an extraordinary and most characteristic 

 feature of the genus, and no trace of any similar structure has 

 been observed in anj^ other crinoid. The precise office of these 

 organs has puzzled all writers hitherto, and must yet remain to 

 some extent a matter of conjecture. Each of the five main 

 trunks contain two longitudinal perforations running a little 

 to either side of the centre, and one of these passes into each of 

 the rounded branches into which the trunk divides, and pierces 

 its centre to the extremity. They have no other opening, and no 

 external furrows of any kind, but are simply ten thickened 

 cylindrical tubes, which join in pairs by their outer walls near the 

 body, where the two together have a transversely flattened 

 elliptic outline. The passages penetrate the body independently, 

 near — and usually somewhat above — the arm openings, though 

 sometimes directly between the openings of the arms of two 

 different rays. Inside the walls they unite with the arm furrows, 

 in a similar manner as the lateral pore passages in Batocrinux, 

 and in view of all the facts, we entertain no doubt, that these 

 openings are of the same nature as tliose which we have called 

 respiratory pores in the Actinocrinida? and Platyci'inida?, and 

 which instead of being simple openings, Avith probably soft parts 

 tittached, are here extended into tubes forming a part of the 

 calcareous test. There are five pairs of these elongated pores, 

 and it makes no material difference whether the}' enter the body 

 a little more radially or interradially, they are placed near the 

 arm bases, and soon connect with the arm passages within the 

 body, as the respirator}- pores in Batocrinus. In many species 

 there are deep depressions in the vault, opposite the base of each 



