No. 3.] GEOLOGY AND MINERALOGY. 347 



tesselated crinoids as the anal tube, corresponding in every respect 

 with the anal tube in Antedon and Fentacrinus, and he maintained 

 the opinion which he formerly published (Edin. New Phil. Jour. 

 Jan. 1861), that the valvular '' pyramid " of the Cystideans is 

 also the anus. The true mouth in the tesselated crinoids is an 

 internal opening vaulted over by the plates of perisom, and situated 

 in the axis of the radial system more or less in advance of the 

 anal tube, in the position assigned by Mr. Billings to his '' am- 

 bulacral opening." Five, ten, or more openings round the edge of 

 the disc lead into channels continuous with the grooves in the 

 ventral surface of the arms, either covered over like the mouth 

 by perisomatic plates, the inner surface of which they more or 

 less impress, and supported beneath by chains of ossicles ; or, in 

 rare cases (^Amplioracrinus), tunnelled in the substance of the 

 greatly thickened walls of the vault. These internal passages, 

 usually reduced in number to five by unitiDg with one another, 

 pass into the internal mouth, into which they doubtless lead the 

 current from the ciliated brachial grooves. 



In connection with different species of Platyceras with various 

 crinoids," over whose anal openings they fix themselves, moulding 

 the edges of their shells to the form of shell of the crinoid, is a 

 case of •' commensualism," in which the mollusc takes advantage 

 for nutrition and respiration of the current passing through the 

 alimentary canal of the echinoderm. Hyponoine Sarsii appears, 

 from Prof. Loven's description, to be a true crinoid, closely allied 

 to Antedon, and does not seem in any way to resemble the Cysti- 

 deans. It has, however, precisely the same arrangement as to its 

 internal radial vessels and mouth which we find in the older cri- 

 noids. It bears the same structural relation to Antedon which 

 Extracrinus bears to Pentacrinus. 



Some examples of different tesselated crinoids from the Burling- 

 ton limestone, most of them procured by Mr. Wachsmuth, and 

 described by Messrs. Meek and "Worthen, show a very remarkable 

 convoluted plate, somewhat in form like the shell of a ScapTiander^ 

 placed vertically in the centre of the cup, in the position occupied 

 by the fibrous axis or columella in Fentacriniis and Antedon' 

 Mr. Billings, the distinguished palaeontologist to the Survey of 

 Canada, in a very valuable paper on the structure of the 

 Crinoidea, Cystidea, and Blastoidea (^SilUnian's Joimial, 

 January, 1870), advocates the view that the plate is connected 

 with the apparatus of respiration, and that it is homologous with 



