146 Kail on the 



prominence according to the position of the branch ; about 

 twenty-four in an inch. Branches often compressed in the direc- 

 tion of the cell to such a degree as to give an apparent double 

 serrature, or serrature on eaoh side of the axis. In this condition 

 the edges of the cells are at right angles to the axis, very shallow, 

 and not pointed. 



When the celluliferous side, compressed in the direction of the 

 cell, is uppermost on the surface of the shale, a line may be traced 

 across the branch joining the edge of the serratures, thus showing 

 that the two apparent serratures are but the single one, so com- 

 pressed that its extremities project beyond the margin. 



We have thus all gradations : the smooth surface of the branch, 

 with minute striations upon the outer side ; the inner side when 

 not compressed with serratures, showing as indented lines across 

 the surface, " i i i i } i ; the double serration, produced by 

 more pressure in the same direction, : ~ ? === ^ 1 ^2'^ I and 

 again, as the branch is turned around, these serratures disappear- 

 ing from one side, and becoming more prominent upon the other 

 C^j^^j^j^j-^j ; finally showing their full breadth as the ray is 

 compressed in its transverse or lateral direction. 



This condition, which has not been understood with regard to 

 many species, is the principal cause of the diminution and some- 

 times final disappearance of cells towards the base of a branch ; 

 even when both sides are serrated, a less degree of compression, 

 which might very naturally result towards the base, would cause 

 the serratures to be less prominent, as is seen in many of the 

 figures in Barrande's Graptolites de Bokeme ; in the New York- 

 Pal ae ontology, etc. 



The serratures of this species differ essentially from those of 

 any other in the Canadian collection, and from any in the New 

 York collections or others that have come under my observation. 



Locality and Formation. — Point Levy, Hudson River Group. 



Collectors, — J. Richardson and E. Billings. 



Graptolithus rigidus. 



Description. — Multibrachiate, bi-lateral ; branches slender, ey- 

 lindroid exteriorly ; rigid, maintaining their width to the third 

 bifurcation, and beyond this very gradually diminishing ; bifurca- 

 tions five in the space of one and a half inches ; internodes unequal 

 shorter near the base, and increasing towards the extremities ; 

 serratures undetermined. 



