476 



BRITISH (IRAPTOLITES. 



Figs. 331 b and c. — Mono gr aphis spiralis 



(Geinitz). 



K'fe 



r 



• 



almost straight are of fairly frequent occurrence, and seem to imply that at its 

 distal extremity the polypary was not convolute but merely somewhat arcuate, as 

 is the case also in M. convolutus. 



The theca?, too, which differ slightly in the proximal and distal portions of 



the polypary, have a still further degree of variation 

 impressed upon them by the conditions of preser- 

 vation. The early theca? are all small, in contact 

 only, and with very definite reflexion of the aper- 

 tural region. This character is maintained in those 

 of the first few whorls of the polypary, and since 

 the theca? steadily become larger and larger, the 

 polypary increases correspondingly in breadth ; but 

 nevertheless the amount of the whole theca involved 

 in the reflexed barb does not increase, in fact the 

 amount of reflexion decreases much in the saint' 

 manner as in M. convolutus. These are the original 

 characters of the thecae, but as the result of the 

 mode of preservation, it not infrequently happens 

 that the thecas of the earlier portion of the polypary 

 have their reflexed apertural regions twisted away 

 from the observer and embedded in the rock matrix, 

 and hence appear merely subtriangular in form, and 

 this may persist throughout the whole extent of the 

 polpary ; in these cases there is, however, other 

 evidence that the appearance presented is not that of a mid-profile view. 



Affinities. — Monog. spiralis presents many points of resemblance to M. convolutus, 

 but may be easily separated by the more persistent widening and greater 

 robustness of the polypary. From M. circularis it differs in the form of the distal 

 thecae. From other species its form is sufficient to distinguish it. 

 Horizon and Localities. — Tarannon Beds (throughout). 



S. Scotland: Belcraig Burn; River Gala, N.W. Buckholm Mill; Thirlstane 

 Score, etc. Wales : Tarannon River ; Road 800 yds. W. of Llawr-y-coed ; Plas- 

 bach Stream, Llanbrynmair; Afon Cwm Caleb, Llanbrynmair, etc.; Nant Fuches- 

 wen, Pont Erwyd. Ireland: Tieveshilly ; Portaferry, Co. Down. Lake District: 

 Pull Beck; Swindale ; Rawthe}' Bridge, Sedbergh. 



Associates, etc.- — Mono//, spiralis is a fairly abundant graptolite throughout the 

 rocks of Gala-Tarannon age; the smaller individuals are usually found in the lower 

 beds, but those from the M. griestoniensis zone are often of considerable size. Its 

 associates are the entire Tarannon fauna. 



Collections. — Geological Survey of Scotland, Sedgwick Museum, Jones, Swan- 

 ston, Lap worth, and the Authors. 



b c 



b. Distal theca'. Enlargement of part 

 of specimen on same slab as PI. 

 XL VIII, fig. 7 c. 



r. Distal thecse, showing different appear- 

 ances j>resented after compression. 

 Ibid. 



