STRUCTUUE OF THE CYSTIDEiE, 2S 



I was delighted to find that he had ah-eady arrived at the same 

 results, and had it in view to prepare a paper upon the subject for 

 the " Transactions of the Geological Society." Upon my informing 

 him however that I was also about to publish the same discovery in 

 this decade, he in the most liberal manner made over his materials 

 to me, and I am thus enabled to give a figure of Actinocrinus rugosus, 

 which shews the course of the ambulacra under the ventral surface. 



The principal difficulty in proving the existence of these orifices 

 is to find specimens so little mutilated at the base of the arms as to 

 exhibit the apertures with their margins uninjured. Hundreds of 

 examples occur with large, irregular openings, but as the edges are 

 fractured all around it is impossible to say whether or not there were 

 originally any natural apertures. It is only in individuals which 

 have ■ been well preserved, and carefully collected and cleaned, that 

 the facts can be clearly observed. In some of the species the 

 apertures are exceedingly small, and so filled with crystalline matter 

 that they can only be seen very indistinctly. In Canjocrimis ornatus 

 (Say), for instance, there are certainly indications of the existence of 

 minute apertures, yet in the best specimens I have seen it would be 

 hazardous to assert it positively. In all the species of Rhodocrinus, 

 Actinocrinus and Platycrinus, the apertures are in general large and 

 easily obsei-ved. Most of the Lower Silurian specimens are in such 

 a condition that nothing can be determined with certainty concerning 

 any of the orifices. In one species from the Chazy and two from 

 the Trenton limestone, all of the genus Hijbocrinus, I have however 

 ascertained their existence. 



The following are some of the species in which I have seen clearly 

 that these apertures actually do penetrate through into the interior 

 of the visceral cavity : — 



1. Eucalyptocr'mus decorus (Phillips). — In this remarkable Crinoid 

 the arms are always found closed into the niche-like, divisions of the 

 proboscis and ventral portion of the cup. It is one of those species 

 whose structure renders a passage for the ambulacral canals through 

 the mouth almost impossible, as the orifice is situated in the apex of 

 a tube that projects above the extremities of the arms. In order to 

 enter the body in that direction, the vessels, after descending the 

 groove on the inside of the arm, could only proceed by climbing the 

 outside of the proboscis, by which course the projecting knob of 

 plates at the top would have to be surmounted. A more mconve- 

 nient route could scarcely be imagined, and we find that nature 

 has provided a much shorter one. While collecting fossils in the 



