248 Stveclish and British Grctjotolitic Deposits. 



a. Trichograptus, Nich. — Frond bilaterally symmetrical, com- 

 pound and branching, composed of two monoprionidian stipes, 

 which proceed from a triangular sicula, and give origin to a series 

 of simple monoprionidian stipes, which arise solely from the celluli- 

 ferous side of the main stipes. No disc. Cellules of the type of 

 Monograptus Nilssoni, Barr. (PI. IX. Fig. 2.) 



This genus is distinguished from Dichograptus proper by the 

 absence of a funicle, and the fact that the secondary branches are 

 given off only from one side of the two main stipes. The frond 

 thus resembles a reflexed Didymograptus, the stipes of which give off 

 branches from one side only, and that from their celluliferous aspect. 

 The type of the genus is T. [Dichograptus) fragilis, Nich., from the 

 Upper Skiddaw Slates (Ann. Nat. Hist., ser. 4, vol. iv. pi. xi. 

 figs. 1-3). The Bichograptus tenellus of Linnarsson, from the 

 " Olenus Schists," also belongs here. 



b. ScHizoGRAPTUS, Nich. — Frond compound, formed of four prin- 

 cipal stipes, which radiate in a cruciform manner from a short 

 central funicle. Each of the primary stipes in turn gives rise to a 

 series of rigid secondary branches, which spring from the same side 

 of the same stipe, nearly at right angles, and apparently do not 

 subdivide. Cellules of the type of Monograptus Sagittarius, His. No 

 disc. (PI. IX. Fig. 3.) The genus may be compared to a form such 

 as Tetragraptus quadrihracliiatus, Hall, in which each of the primary 

 stipes gives off branches on one side. The type of the genus is S. 

 (JDicJiograptus) reticulatus, Nich., from the Lower Skiddaw Slates 

 (Quart. Journ. Geol. Soc, vol. xxiii. pi. v. figs. 4 and 5). 



c. Temnograptus, Nich. — Frond branching and rebranching; 

 composed of four primary stipes, which radiate from a short central 

 funicle, and then divide repeatedly and dicliotomously at nearly 

 regular intervals. Cellules apparently of the type of Monograptus 

 Sagittarius. No disc. (PI. IX. Fig. 1.) 



The frond in this genus may be compared to a Tetragraptus, in 

 which each of the four main stipes divides dicliotomously, the branches 

 thus produced dividing repeatedly in a similar manner. The type of 

 the genus is T. (Bichograptus) midtiplex, Nich., from the Lower 

 Skiddaw Series (Quart. Journ. Geol. Soc, vol. xxiii. pi. vi. figs. 1-3) . 

 The generic distinctness of this type has previously been recognized 

 by Mr. Lapworth (Geol. Mag., Dec. 1873) ; and it is probable that 

 Graptolithus Milesi, Hall, may be regarded as belonging to this genus. 



d. Ctenograptus, Nich. — Frond branching and rebranching. 

 Cellules extraordinarily close-set, the cell-mouths running directly 

 across the stipes, as so many linear slits. Base unknown. The 

 type of this genus (which I have only established provisionally) is 

 C. (Bichograptus) annidatus, Nich., from the Lower Skiddaw Slates 

 (Ann. Nat. Hist., ser. 4, vol. ii. pi. xi. figs. 4 and 5). From the 

 characters of the cellules it is certain that this form must be separated 

 from Bichograptus proper, from which its mode of branching also 

 distinguishes it ; but the materials in my hands are not sufficient 

 for a complete or satisfactory diagnosis. (PI. IX. Fig. 4.) 



For the better understanding of the form of the four above- 



