/. Hopkinson — On a New Genus of Graptolites. 23 



The genus Dicellograpsus is exclusively Lower Silurian, and in our 

 Lower Silurians is eminently characteristic of the Llandeilo forma- 

 tion, as developed in the south of Scotland. Not a single species 

 has hitherto been detected in the Arenig of the British Isles or other 

 countries, in the Llandeilo of Wales, nor in the Caradoc of West- 

 moreland, Wales, or Scotland; while in the Llandeilo rocks of 

 Dumfriesshire, which are most probably very near the base of the 

 Caradoc, all its species are found. Two species occur in Ireland, 

 associated witb other Llandeilo graptolites, the general fauna being 

 of Caradoc age. One, D. Moffatensis, occurs in the Caradoc of New 

 York, and another, D. ForcMnmmeri, the earliest known species, was 

 originally described from the Island of Bornholm, where it occurs in 

 strata which most probably represent the Caradoc formation. It 

 seems, therefore, most probable that the genus first appeared in tbe 

 locality where it most abounds — where it is both specifically and 

 numerically abundant — and that from this locality (Dumfriesshire) 

 it has migrated elsewhere. 



The genus comprises the following species : — 



1. Dicellograpsus ForcMammeri, Gein., sj). Cladograpsus ForcTi- 

 hammeri, Gein. (1852), Die Graptolithen, p. 31, pi. v., fig. 28-31. 

 Didymograpsus Forchhammeri, Baily (1862), Expl. Sh. 133, Geol. 

 Surv. IreL, p. 14, fig. 6. Journ. Geol. Soc. Dubl., vol. ix., p. 305, 

 pi. iv., fig. 7.— PL I., Fig. 1. 



Branches of polypary nearly straight, diverging from each other 

 at a wide angle, and with a long and slender axillary spine. 

 HydrothecEe about 25 to the inch ; two or three times as long as the 

 width of th.e polypary, and free for but a small portion of their 

 length. Eadicle and lateral spines very minute. 



Tlie long and slender branches, Tarying in different indiriduals from l-SOth to 

 l-30th of an inch, broad at their origin, gradually increase until a breadth of fully 

 l-20th of an inch is attained; in the longest, and therefore oldest, individuals, 

 becoming narrower towards the distal end. They diverge at an angle of from 90 to 

 120 degrees, and attain a length of several inches. The axillary spine is always well 

 developed, is frequently more than l-20th of an inch long, and averages l-200th in 

 breadth. The radicle is usually very minute, but in some specimens it ends abruptly 

 without coming to a point, appearing as if it had been broken. The very slender 

 lateral spines proceed from near the apex of the first formed thecse, and diverge from 

 each other at a wide angle. The hydrothecse, which are very similar in all the 

 species of this genus, are for a very short distance free, and this free portion is closely 

 appressed to the body of the polypary, and sometimes very much incurved. At about 

 half their length they are slightly indented, and opposite this depression the back of 

 the polypary is slightly enlarged, giving it a waved appearance. 



The persistent and usually very conspicuous axillary spine should suffice to prevent this 

 species being considered identical with D. Moffatensis. But as yet D. Forchhammeri 

 has never been correctly figured, or fully described. Neither from Geinitz' nor from 

 Baily's figures (all we have), could the character of the proximal end be determined. 

 Three of Geinitz' figures (29, 30, and 31) show the axillary spine ; in the other (28) 

 it is not shown ; but this is only a restoration from a single branch, according to 

 Geinitz himself.^ The enlarged drawing (28«), intended to show the structure of the 

 hydrothecse, is incorrect. The " round eyes," which are figured and described as the 

 apertures of the hydrothecse, are merely the indentations between each theca ; the 

 apertures themselves, though opening out towards these indentations, are not visible. 

 Mr. Baily appears to have had the same idea of this structure as Geinitz, 



^ Die Grapt., expl. pi. v. fig. 28. 



