128 Carruthers — On British GraptoUtes. 



Polypary growing in one direction from the proximal end, and 

 giving off simple or compound branches at irregular intervals. 



Sp. I. C. Mn7rhisonii, s^. nov. (Table V. Y\g.\']a, \'jb). Hydrothecae triangular 

 apiculate, furnished with a spine. The upper margin of the cell at right angles to 

 the axis, about twenty-eight cells to the inch. The polypary is considerably 

 incurved at its proximal end, and as it grows it gradually opens into a larger curve. 

 The branches spring from celluliferous surface of the polypary, but as there is no 

 break in the continuity of the hydrothecse, they must rise from the periderm 

 covering the common canal. The branches also curve in the same direction as 

 the main portion of the polypary. Loc. Pencerrig, Builth. I have associated 

 the name of the author of "Siluria" with this remarkable species. The only British 

 specimens I have seen are in the Geological Museum, Jermyn Street, but among 

 the specimens obtained by the British Museum from M. Barrande there is a 

 specimen from Listice, labelled G. priodon., which belongs to this species, 



2. C. hamatus {G. hamatus, Bail. I. Quart. Journ. Geol. Soc. Dubl., i86i, pi. 4, 

 fig. 6). This remarkable form, of which I believe only a single specimen has been 

 found, now in the Museum, Jermyn Street, where I have examined it, probably 

 belongs to this genus. 



Gen. IV. DiDYMOGEAPSUs, M'Coy (Brit. Pal. Fossils, p. 9). Poly- 

 pary growing bilaterally from the initial point, consisting of two 

 simple or bifurcate branches and without a central disc. The initial 

 portion of the proximal end forming a non-celluliferous process always 

 proceeding from the common canal or axis ; the opposite or celluli- 

 ferous side of the polypary frequently ornamented with one or more 

 teeth or spines. The branches of the polypary sometimes extend at 

 right angles to the initial process (D. hirundo), sometimes they are 

 bent backwards upon it (2). Moffatensis), and less frequently they 

 are turned inwards from it {D. Murchisonii). 



The forms with bifurcating branches for which Salter proposed the genus Teira- 

 grapsus do not differ in any essential character from the type of Didyjnograpsus. 

 The precise number of branches is not taken into account in his genus Dichograpsus, 

 in which there are species with eight, eighteen, etc, , branches ; and if of no value 

 there, it can scarcely be employed here. It is not easy indeed to discover any 

 character whereby to separate Dichograpsus from this genus, except it be the cor- 

 neous disc that envelopes the non-celluliferous proximal portions of the polypary, 

 which has never been found associated with specimens of Didymograpsus, and 

 only with certain forms of TeU-agrapsus and Dichograpsus. But this disc is not 

 always present even in those graptolites in which it is known to occur ; it may have 

 been more perishable than the polypary itself; perhaps, however, the species which 

 originally possessed it may be distinguished by other characters, as by the posses- 

 sion of an obvious branching hydrocaulus. There are, unfortunately, no materials 

 in this country to enable one to determine this ; but if this character should prove 

 a good one, it would enable us to class the various forms into two well marked 

 genera ; some species of Tetragrapsus, such as G. (T.) Headi and G. crucifer, 

 belonging to Dichograpsus^ while G. (T.) bryonoides^ etc., would be placed in 

 Didymog rapsus. 



Sp. I. D. hirundo^ Salt. (Quart, Journ. xix. p. T37, fig, 13/"), D. constrictus^ 

 Hall (Grapt. Quebec Gr. p. 76). Polypary of two branches, diverging at right 

 angles from the initial points, and having its full size from the beginning. From 

 22 to 26 cells in an inch. Loc. Skiddaw Slates. 



2. D. Murchisonii, M'Coy, Graptolithus Murchisonii, Beck (Sil. Syst. p, 694, 

 pi, 26, fig. 4). Prionotus geminus. His, (Leth, Suec. Suppl, 2, p. 5, pi. 38, fig. 3). 

 Loc, Llandrindod, etc. 



3. Z>. V-fractus, Salt. (Quart. Journ, xix. p. 137, fig. I3<f). Polypary of two 

 branches bent inwards, forming an acute angle but speedily opening, and widely 

 diverging from each other. From 20 to 24 cells in an inch. Loc. Skiddaw 

 Slates, 



The amount and direction of divergence of the branches are so variable in 

 some well-marked species of this genus, as might be expected in polyparies com- 



