486 ON CAEBONIFEEOUS ENCETNITES. 



In the fossil ciinoids Mr. Rofe has shown that the only- 

 difference is, that the current enters the ,body at the base of the 

 arms and passes to the central cavity through covered channels 

 under the dome, instead of in open grooves on the top of it. 



As the genus Gilbertsocrinus has no proboscis, there is a danger of 

 the water which has passed through the body being immediately- 

 returned to it by the arms. This difficulty is met by the peculiar 

 structure I have described. In the first place, the groove in the 

 arms is not continued to the base, but becomes a central passage. 

 This passage is, further, very small, so that only a limited quantity 

 of water passes in that way. The arms, again, are set on at right 

 angles to the body so as to be as far as possible out of the way of 

 the issuing water. In some cases the arms contract in size very 

 remarkably immediately above the base. To meet the deficiency in 

 the supply of water through the arms these openings are made below, 

 and the current from the arms passing over the top of these openings, 

 draws in, mechanically, a supply of fresh water from below, which 

 is further protected from mixing with the issuing water by the 

 over-hanging roof of the arm-bases. This explanation seems 

 preferable to the other because it is unlikely that this genus only 

 should be provided with ovarian apertures, while the absence of a 

 proboscis gives a definite reason why this genus should find it 

 advantageous to have these subsidiary openings. That the 

 perforations in the arms are really ambulacral, and are not for the 

 passage of muscles as has been suggested, seems to me clear from 

 the fact that if they were filled with muscle there would be no 

 means by which the current set up by the tentacles could reach the 

 interior, as there is no external groove. 



The genus Gilbertsocrinus may now be thus defined — basals 5 ; 

 sub-radials 5 j radials 3 ; brachials several, generally irregular ; the 

 second brachial channelled at top, and leading into an orifice 

 which communicates with the perforation in the arms; axillary 

 plates well developed ; arms round, and generally set at right angles 

 to the body ; plates of body generally tuberculate. It will include 

 the following species — lursa (Phillips), alnormis (WcQoj)^ calcaratus 



