380 



BRITISH GRAPTOLITES. 



tumescens, Wood. 



Ai 



maxinuini ))i'ca(ltli of 2 nun. Tlieca) typically eleven to nine in 10 mm., 



the matnre ones abont 3 mm. in length, fonr times as long as wide and 



overlapping one-half to two-thirds their length; average inclination oO''. 



Bescrij)tion. — The polyi)ary is always distinctl^^ incurved in the })roximar 



portion; it attains its maximum Ijreadth rapidly, usually l)et\veen the seventh and 



tenth thecas, and thereafter may show slight decrease in breadth np to the distal 



extremity ; this, however, is not a constant feature. Certain forms from the Lake 



District are rather shorter and broader than those from Wales and the Welsh 



Borderland and have more closely set thecne (thirteen to twelve in 10 mm.). 



The sicula has a length of rather less than 2 mm., and is about five times as 

 Fios.2i9aandh.-Monognrpias l*'".^^ '^^ ^idc ; it appears to rcacli nearly to the 



upper limit of the second thecal aperture. 



The thecfB vary somewhat in number in speci- 

 mens from different localities, but have invariably 

 the same general characters, and usually show a 

 somewhat thickened apertural margin, which after 

 compression gives rise to a distinct denticle. The 

 ])roximal thecas are usually somewhat shorter than 

 those of the distal portion of the polypary. 



Aflinifu's. — Monoij. tumcscenH is one of the many 

 forms to which the name M. culonns has been 

 erroneously assigned. It is, however, clear that it 

 differs widely from that species in the characters 

 of the theca3 of the proximal portion. From M. 

 (Inhins, with which it agrees in many I'espects, it 

 wgemeut of part of PI. xxxvii, differs in the rapid widening of the polypary and 



in the possession of longer, more overlapping tliecEB ; 

 Avhile from iV. viihjdrix, with which it agrees fairly in the character of the thecse, 

 it differs in the form of the proximal end and of the polypary in general. 



Horizon and Localities. — Lower Ludlow Shales (especially zone of M. tumescens). 

 Montgomer i/sJiire : Llettygynfach (south side of Long Mountain) ; Montgomery 

 Road. Lndlow District : Elton Lane ; Gorsty Farm, Elton-Ludlow Road ; near 

 Stormer Hall. Lake District : Dent. 



Associates, etc. — Monog. tumescens is a fairly abundant form in the Lower 

 Ludlow Shales, especially in a belt of rocks near the top, where it occurs associated 

 Avith M. tumescens, A'ar. minor ; it also occurs xory occasional!}^ at loAver horizons 

 associated with il/. l/oJioiiicvs and M. cliivncra. 



Cotlectiorts. — Sedgwick Museum, La})wortli, and the Authors. 





a. Proximal end, showing- sieula. En- 



largoinent of part of PI. XXXVII 

 fijj. 12 <(.. 



b. Distal end, showinsr virtfula. En- 



