from the Quebec Group of Point Levis. 137 



twentieth of an inch ; width of stipe at commencement one 

 twenty-fifth of an inch, at widest portion about one line ; total 

 length of frond unknown, but exceeding three inches and a 

 half; distance between the stipes on opposite sides of the frond 

 from one fifth to one fourth of an inch, except close to the 

 funicle. 



Cellules about twenty-five in the space of an inch, inclined 

 to the axis at about 45°; the denticles prominent and sharply 

 pointed or submucronate ; the cell-mouths curved at right 

 angles or nearly so to the cellules, and making an angle of 

 about 135° with the axis (fig. 2, h). 



Tetragrapsus apiproximatus is most nearly allied to T. cru- 

 cialis, Baiter { = Gra2)toIithus quadrtbrachiatus, Hall), from 

 which, however, it is separated by several very important 

 peculiarities. Most striking amongst these is the very remark- 

 able shape of the frond. In T. quadrih-ackiatus, Hall, when 

 undistorted,the stipes upon the same side of the funicle are nearly 

 at right angles to one another ; so that (keeping the funicle 

 horizontal) the left-hand upper stipe forms nearly a straight 

 line with the right-hand lower stipe, and the other two stipes 

 similarly form a straight line. The whole frond, therefore,, 

 has in this species very nearly the shape of the letter X ; and 

 it may be compared to what would result if two examples of 

 Didy7nograpsus serratuluSj Hall, were united back to back by 

 their radicles. In Tetragrap)sus ai^proximatus^ on the other 

 hand, the two stipes on the same side of the funicle (keeping 

 the funicle, as before, in a horizontal position) are nearly in the 

 same straight line, and the two stipes on the one side are, as 

 nearly as may be, parallel with those on the other side. Hence 

 the whole frond (and this is a fact worthy of notice) bears a 

 very close resemblance to two individuals of Bidymograpsus 

 patidus, Hall, united back to back by their radicles, this re- 

 semblance being increased by the similarity in the shape of the 

 cellules in the two species. 



Again, the cellules in Tetragrapsus approximatus are much 

 more highly inclined to the axis than they are in T. quadri- 

 hrachiatuSj the denticles are much more prominent and pointed, 

 and the cell-mouths are markedly curved instead of being 

 straight. As in T. quadribrackiatus^ the funicle does not appear 

 to have been embraced by a central corneous disk. The pecu- 

 liarities above mentioned as distinguishing T. approximatus 

 are constant in a large number of individuals ; and therefore 

 no doubt can be entertained as to the distinctness of the 

 species. 



Loc. and Form. Common in dark grey or greenish grey 

 shales of the Quebec group, Point L(5vis. 



