Mr. C. Lap worth on new British Graptolites. 171 



is usually a gradual increment in width from the initial ex- 

 tremity of the polypary to its distal termination ; but occa- 

 sionally the margins in the distal moiety are perfectly 

 parallel. 



The test is of great tenuity, and is frequently invisible ; an 

 outline only of the angles of the fossil is preserved in 

 strong chitinous threads. The zigzag septal line seems to be 

 formed of a single thread of this nature, apparently of no 

 greater thickness than that which outlines the hydrothecae. 

 The apertural spines occasionally attain a length equal to 

 three fourths of the diameter of the polypary itself, and have 

 almost invariably an upward direction. 



This species is most intimately allied to the Glenkiln form 

 I have usually referred to Diplograptus (Idiograptus) Whit- 

 fieldii (Hall). In that species, however, the thecae are more 

 distant, and the virgula is greatly prolonged, both proximally 

 and distally. (Comp. Lapworth, Graptolites County Down, 

 Proc. Belfast Nat. Field-Club, 1876-7, pi. vi. fig. 21.) 



Horizon and Locality. Common in a single zone in the 

 Middle Bala beds of Girvan at Shalloch Mill &c. Rare in 

 the Hartfell Shales upon the same horizon at Syart Law, 

 Dobb's Linn, &c. 



28. Diplograptus (Cyrtograptus) tricornis, Carr. 

 (PI. V. figs. 27 a-27 e.) 



In 1858 Mr. W. Carruthers, F.R.S., described and figured 

 a remarkable diprionidian Graptolite from the Moffat Shales 

 of Hartfell Spa, under the title of Diplograpsus tricornis 

 (Carruthers, Trans. Roy. Phys. Soc. Edinb. 1858, p. 468, 

 fig. 2). The chief feature relied upon by its founder as 

 affording conclusive evidence of its distinctness from its 

 frequent associate Diplograptus foliaceus, Murch., was the 

 presence of three strong spines at the proximal extremity of 

 the polypary, the central spine being perpendicular, and 

 the two side spines approximately horizontal. The illustra- 

 tive figure, however, shows a far more remarkable peculiarity • 

 the hydrothecoa are exhibited as rhomboidal in form, in con- 

 tact throughout the whole of their extent, their bases resting 

 upon the septal line, and their apertures (?) forming a con- 

 tinuous and slightly undulating line along the ventral margin 

 of the polypary. 



The following year Professor Hall gave a diagnosis and 

 figure of an American species from the Hudson-River group 

 under the title of Graptolithus marcidus (Hall, Pal. New 

 York, iii. p. 515, figs. 1-3). Like Mr. Carruthers's species, 

 this new form was provided proximally with two horizontal 



