230 



IJUlTlSll (UIAPTOLITES. 



Pias. 151 d -f.—Orthograptus vesicu- 



lusiis, Nicholson. 





d 





I 



V,: 



> i 

 1 ■'/ 



Various stages in the development of 

 the young polypary, stowing great 

 length of sicula and origin of th. I 1 . 



d. On same slab as 150 b. 



e. Dobb's Linn. Coll. H. M. Geological 



Survey, Edinburgh. 

 /. Beleraig Burn. Coll. Wood. 



The sicula appears to be at least mm. in length, 

 l)iit it is hard to determine this with accuracy, 

 either because it passes so gradually into the 

 virgula, or else because it is enclosed in the mem- 

 brane which later expands into the vesicle. In any 

 case it can only be detected in young individuals, 

 since the apex is concealed by the theca3 in all adult 

 forms and is only partially visible even on the obverse 

 side. The virgella in some Irish specimens measures 

 1*2 cm. in length, but in Scottish and English speci- 

 mens it is as a rule considerably shorter, though in 

 one Scottish specimen it measures 1*3 cm. Th. I 1 

 originates about 3 — 4 mm. above the aperture of the 

 sicula and grows downward below it before bending 

 upward and outward ; th. I 2 crosses the sicula 

 nearly horizontally, so as to conceal it almost com- 

 pletely in the reverse aspect. The septum is de- 

 veloped after two pairs of thecae have arisen, and 

 seems to be complete. Compressed, pseudo-" bi- 

 scalariform" views (PI. XXVIII, fig. 8 c) of the 

 polypary are of common occurrence and charac- 

 teristic of the species. It is only rarely that the thecal tubes are clearly seen ; 

 they have a general inclination of 20° — 25°, and a length of 2 mm., and are 

 overlapped for one half of this. They are broad for their length and have wide 

 apertures, occupying about one half of the total breadth of the polypary. 



Affinities. — 0. vesiculosus differs from its variety var. penna, to which it is 

 very closely allied, in being a longer form and in having a larger and more 

 conspicuous vesicle. It is also allied in some aspects to Mesog. modest us, but 

 differs from that species in the possession of a vesicle and in having larger thecse. 

 Horizon and Localities. — -Llandovery (Birkhill Shales) (zone of 0. vesiculosus). 

 8. Scotland : Dobb's Linn ; Frenchland Burn ; Beleraig, etc. Ireland: Coalpit 

 Bay, Donaghadee ; Little River, Pomeroy, Co. Tyrone. Lake District : Skelgill. 



Associates, etc. — 0. vesiculosus is usually a fairly abundant fossil wherever the 

 Lower Birkhill Shales or their equivalents are developed. Some very fine Irish 

 specimens from Donaghadee are in the possession of Mr. Swanston, of Belfast, and 

 in the Museum of the Belfast Natural History Society. A few specimens only are 

 known from the Lake District and they are mostly small and poor. The species 

 occurs in Scotland and Ireland associated with Monog. tenuis, Mesog. modestus, 

 < 'liuiacog.medius and various Dimorphograpti, including D. confertus, and D. Siranstoni. 

 Collections. — Swanston, Museum of Belfast Natural History Society, Geolo- 

 gical Survey of Scotland, Sedgwick Museum, Lapworth, and the Authors. 



