species o/" Didymograpsus. 349 



Didymograpsus Murchisoni^ Beck, sp. PI. VII. figs. 7, la^Th. 



Graptolithus Murchisoni, Beck (Sil. Syst. p. 694, pi. 26, fig. 4). 

 Graptolites Mtirchisoni, M'Coy (Pal. Foss. ii. p. 5). 



Frond consisting of two stipes springing from a mucronate 

 base, and including between them an angle of divergence of 

 from 10° to 15° or 20°. The stipes vary in length from a 

 quarter of an inch up to two inches or more, proceeding from 

 the radicle outwards and upwards with a slight curve, and 

 being then continued to their terminations nearly in straight 

 lines. The widtli of the stipes varies greatly in diiferent in- 

 dividuals ; but they are always narrowest at the base, expand 

 gradually till their full width is attained, and then gradually 

 contract towards their distal extremities. The back of the 

 stipe, however, is never so straight as in typical examples of 

 D. bifidus, Hall, and the celluliferous margin is not so strongly 

 convex. Specimens of average size have a breadth near the 

 base of one twenty -fourth of an inch, and in the fully-developed 

 portion of from one to one and a half line. Gigantic indivi- 

 duals, however, not unfrcquently occur (fig. 7 a) in which 

 these same measurements are one line and a half and one- 

 quarter of an inch respectively ; and even these limits are 

 occasionally exceeded. The base is obtusely pointed, and is 

 furnished with a long triangular mucro or radicle, the length 

 of which is from one to one and a half line. In the large 

 specimens, however, the radicle is much less developed pro- 

 portionally, and is blunt and obtuse. The cellules are on the 

 opposite side of the frond to the radicle, or occupy the sides of 

 the angle of divergence, and are from twenty-two to thirty-tAvo 

 in the space of an inch, having the proximal lip of the cell-aper- 

 tures prolonged into long acute denticles. In the smaller speci- 

 mens the cellules form an angle of about 45° with the axis, are 

 free for about half their entire length, and have the cell-mouths 

 somewhat curved and nearly rectangular to the axis. In the 

 larger specimens, the cellules in the fully-developed portion of 

 the stipe lose many of these characters, becoming more nearly 

 horizontal or rectangular to the axis, whilst they overlap one 

 another throughout the greater part of their length, and have 

 the cell-apertures directed decidedly downwards, owing to the 

 great prolongation of the proximal margin of each. 



Hall Tetragrapsus {Graptolithus) bri/onoides, Salter's specific name should 

 have the priority, as bearing the date 1853, whereas Hall's name was 

 given in 18-57. The species, therefore, should be called Tetragrajjms 

 cachtcevs, Salt., sp. There appears, however, to be no doubt that the form 

 is really identical with the Fticoides sert'a of Brongniart, published in 

 1828. In strict justice, therefore, the species should be called Tetra- 

 grapsus serra, Brongn., sp. 



