

MONOGRAPTUS. fc37 



rather more than one-half the total breadth of the polypary. Overlap 

 insignificant. 

 Description. — The polypary is so slender in its proximal portion that little else 

 than the short thecal hooks can be made out; these number from six to seven in 



10 mm. As, however, the polypary increases in 



Figs. 300 a — c. — Monograptus gem- ■* -,,, . , -, . .. ', , . 



matus (Barrande). breadth the adnate region or the theca becomes 



proportionately more evident, and the theca as a 



whole becomes shorter and. broader: the apertnral 



region is a more pronounced hook, the apical extre- 



J mify being bent over to run approximately parallel 



with the general direction of growth of the polypary. 



fe» The torsion of the thecal axis results in the almost 



entire concealment of the hook in some views of the 



polypary. 



Affinities. — There appears to have been consider- 

 ed Distal theca}. Enlargement of part , , r> • t i ■ • i,. 



of Pi. xliii, fig. 5 o. able contusion regarding this species, resulting no 



/'. Thecae in full relief, with apertures i -i. /-i \ j> ,-\ p , i i 



embedded. Enlargement of part doubt 0) h ' om the extreme tenuity of the polypary, 

 c D^£f&bJ form of aper- ( 2 ) £rom tlie difference iii the shapes of the thecal 



XLifi £ ni *J emerA of part of PL hooks in .its proximal and distal portions, (3) the 



failure to recognise and interpret the different 

 appearances produced as the result of torsion of the thecal axis. M. gemmatus 

 resembles in several respects other thread-like Monograpti, such as M. Barrandei, 

 etc., but from these it may be distinguished by the nature of the apertnral 

 terminations, which are definite hooks in M. gemmatus, but lobes in the other 

 species. From M. crinitus, another thread-like species, it differs in the characters 

 of the thecas, which are also rather more distant. 



Horizon and, Localities. — Llandovery (Birkhill Shales), zones of M. gregarius, 

 M. convolutus, and M. Sedgwiclcii. 



S. Scotland : Dobb's Linn ; Belcraig Burn ; Duffkinnel ; Frenchland Burn ; 

 Kirk Gill ; Ettrick Bridge End, etc. Wales: Conway; Parys Mountain. Lake 

 District: Skelgill. Ireland: Coalpit Bay, Donaghadee. 



A-ssoriatfs, etc. — Month/, gemmatus is a fairly common fossil in the higher parts 

 of the Birkhill Shales in S. Scotland, occurring in the three highest zones in 

 association with all their commoner graptolites. At Skelgill it occurs in the sub- 

 zone of M. fimbriates, associated with the zone fossil, Rastrites approximates, and 

 M. triangulatus. 



Collections. — Geological Survey of Scotland, Sedgwick Museum, Lapworth, and 

 the Authors. 



IV. A. 2. (/>) : Monograpli in which the theca? are uniform and the polypary 



shows spiral curvature. 



