Yol. 54.] 



rATTNA OF THE SKIDDAW SLATES. 



633 



Didymograptus indentus var. nanus is a peculiar modification of 

 D. indentus (Hall), from which it is, no doubt, derived. 



(/3) Group containing Bryograptus ramosus var. cumbrensis, 

 Tetragrapius fruticosus, and Didymograjptus furcillatus. 

 With regard to Tetragraptus fruticosus (Hall) and Dtdymo- 

 graptus furcillatus, Lapw., the resemblance is not quite so close ; 

 nevertheless, the differences are so slight that Didymograptus 

 furcillatus may well be developed from Tetragraptus fruticosus, 

 with other slight modifications to suit special conditions. It is 

 not reasonable to expect that we should in every case have the 

 simplicity shown in Group a. The simplification in the branching 

 takes place in the same way as in that group. 



Species. 



Cell-characters, etc. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 

 tural 

 angle. 



Over- 

 lap. 



less ^ 

 1 2 



1 



Bryoyraptus ramosus var. 



cumbrensis. 

 Tetragraptus fruticosus. 



Didymograptus furcil- 

 latus. 



f Gradual increase 

 in width of stipes 



-( from origin. Cells 

 curved, apertures 



1^ concave. 



24 (9-10) 

 15(6) 



25 (10) 



20° 

 38° 



45° 



115°- 

 120° 

 90° 



90° 



(y) Group containing Bryograptus ramosus var. cumbrensis, 

 Tetragraptus Postlethiuaitii, and Didymograptus bijldus. 



It seems likely that Didymograptus bifidus (Hall) may have 

 been similarly developed from Bryograjjtus ramosus var. cumbrensis 

 through Tetragraptus Postlethwaitii by a process of simplification of 

 the branching accompanied by some other slight modifications, such 

 as gradual increase in the number of cells in a given unit of length, 

 concurrent with a gradual increase in the angle at which the cells 

 are inclined. 



Species. 



Cell- characters, etc. 



No. of 



cells 



to inch. 



Incli- 

 nation. 



Aper- 

 tural 

 angle 



Over- 

 lap. 



Bryograptus ramosus var, 

 cumbrensis. 



Tetragraptus Postle- 

 thwaitii. 



Didymograptus bifidus. 



/'Stipes of general 

 } uniform width. 

 ! Cells slightly 

 1 curved, with 

 1 slightly concave 

 1^ apertures. 



24 (9-10) 



30 (12) 



32-36 

 (13-14) 



20° 

 30° 



45° 



115°- 

 120° 

 130° 



oblique 



less ^ 



{I) Group containing forms derived from Tetragraptus Bigsbyi. 



1 feel certain that Tetragraptus Bigsbyi (Hall) has been deve- 

 loped from a species of Bryograptus in a manner similar to that 

 described above for other forms of Tetragraptus, but the particular 



