646 J. HOPKINSON AND C. LAPWOETH ON THE GEAPTOLITES OF 



the same straight line, except in the immediate neighbourhood 

 of the axil, where they are slightly curved, diverging from a 

 well-marked sicula with a primary angle of about 210°, and 

 increasing in width from one fortieth to one twelfth of an inch 

 in the first six inches of their length ; hydrothecaB from 16 to 18 

 to the inch, inclined at an angle of from 30° to 40° to the axis 

 of the branch ; apertural margin concave, and forming r with the 

 slightly curved outer margin, a prominent and almost equi- 

 lateral denticle. 

 Of all our British species this form most nearly approaches the 

 D. serratulus of Professor Hall (Pal. New York, vol. i. pi. 74. figs. 

 5a, 56). It resembles his example in the remarkable rigidity of its 

 branches, and presents us with exactly the same number of thecse to 

 the inch. It also occurs in association with D. superstes, Lapw.MS., 

 at Abereiddy Bay, as in his locality (Norman's Kill) and in South 

 Scotland. From his description, however, we gather that the branches 

 of D. serratulus are straight from their origin, and are almost fili- 

 form throughout, definitions it is impossible to apply to those of the 

 present species. The thecse of his figured example are also of very 

 different form, and greatly resemble those of D. swperstes. 



The more prominent characteristics of Didymograptus euoclus 

 are the small curved proximal portions of the branches, their ex- 

 traordinary length and insensible augmentation in diameter, and 

 the very distant peculiarly shaped hydrothecse. 

 Loc. Lower Llandeilo, Abereiddy Bay. 



Didymogeaptus bifidus, Hall, sp. PL XXXIII. figs. 8 a-S e. 



1865. Graptolithus bifidus, Hall, Grapt. Quebec Group, p. 73, pi. i. 



figs. 16-18, pi. iii. figs. 9, 10. 

 1868. Didymograpsus bijidus, Nich. Quart. Journ. Geol. Soc. vol. 



xxiv. p. 136. 

 1870. Didymograpsus bifidus, Nich. Ann. & Mag. Nat. Hist., scr. iv. 



vol. v. p. 346, fig. 7. 



Branches from half an inch to one inch in length, very narrow at 

 their origin, but gradually expanding to about a tenth of 

 an inch or more in width, and more rapidly contracting towards 

 their distal end, their dorsal margin being nearly straight and 

 their ventral curved ; diverging from each other at an angle of 

 about 300 degrees near their origin, increasing to 340 degrees, 

 which they maintain throughout their length ; sicula slender, 

 gradually tapering to a fine point ; hydrothecas from 32 to 36 

 to the inch, slightly curved, inclined to the axis of the branches 

 at an angle of about 45 degrees, three or four times as long 

 as wide, and free for about one fourth their length ; apertural 

 margin concave, forming an obtuse angle with the axis. 



There occur in the St. -David's beds several forms of Didymo- 

 graptus, in all of which the angle of divergence is so large that the 

 ventral margins of the branches are parallel, or but slightly diver- 



