22 CANADIAN FOSSILS. 



Explanation of Figckes. Plate I. 

 Figures la-lh. Deltoid plates of B. car char icedens. 



" li. Side view of a specimen which has three of the rays partly preserved. 

 " Ik. The same, seen from above. 



" ll. Vertical section, shewing the position of the column in the interior. 

 " Im. Fragment of the base, shewing the manner in which the column 



penetrates into the visceral cavity. 

 " In. Under side of Im. 

 " lo-ls. Small pentagonal crinoidal remains found associated with B. 



carcharmdens. 



Locality and formation. — Caughnawaga, Island of Montreal, Isle 

 Jesus, Isle Bizard. Chazy limestone. 



Pachyocrinus crassibasalis, Billings. 



Plate I. Figures 3a, 3&. 



Description. — Of the species for which the above name is proposed 

 no perfect specimen has been found, and yet we have sufficient to 

 shew that it belongs to a new genus. There are five small penta- 

 gonal basal plates concealed within the cavity for the attachment 

 of the column. Above these and alternating with them are five 

 very large and thick plates, which may be either sub-radials or 

 first-radials. The lower portions of these plates are curved under 

 the body, so as to constitute a broad, rounded or concave bottom to 

 the cup, w^hich has a width of nine lines at a height of about two 

 lines. At this level the cup is broken ofi* in the only specimen 

 discovered, and we have no means of determining what was the 

 structure above. There are no other Crinoids in the Lower Silurian 

 rocks with a base like this, for although in the genus Rhodocrinus 

 some of the species have a concave bottom, yet the sub-radial plates 

 are always small and thin. It is true that mere difference in the size 

 of organs or portions of an organic structure is not always of generic 

 value; yet, where it is so extremely great as it is between the 

 sub-radials of Rhodocrinus and the second series of this species, it is 

 generally found that other differences exist sufficient to warrant a 

 separation of the species in which such differences occur into distinct 

 genera. I am inclined to believe, therefore, that when perfect spe- 

 cimens are found they will shew that this is a new genus. 

 E.\planation op Figures. Platb I. 

 Figure 3a. View of the base of the specimen. 

 " 3b. Side view of the same. 



Locality and formation. — Two miles north of Montreal, in the 

 Chazy limestone. 



