MONOGRAPTUS. t39 



Horizon and Localities. — Lower Gala-Tarannon Hlh\*. 



Wales: Forge Corner, Conway; Llanystwmdwy ; Tarannon River; (Jelli- 

 dywyll Stream; Grelli Stream; Pontbren-dibyn, near Llanbryninair, etc. 



S.Scotland: Belcraig Burn ; Ladliope ; Bargany Pond Burn; Buckholmside, 

 Galashiels, etc. Lalce District .• Swindale Beck ; Spengill ; Rawthey Bridge, 

 Sedbergb, etc. Ireland ■■ Tieveshilly. 



Associates, etc. — Monog. turriculatus ranges throughout the whole thickness 

 of the Lower Tarannon Shales, but is comparatively rare in the Rastrites maximus 

 band at the base of the zone of M. turriculatus, where it occurs in association with 

 R. maximus, Petalog. altissimus, Monog. nudus, M. dextrorsns, M. Halli, M. runcinatus 

 and other forms. It is far commoner in the higher parts of the zone of M. turri- 

 culatus, where it is found with M. runcinatus, M. Karri, M. nodifer, etc., but 

 becomes rarer again in the higher zone of M. rrispns, where it occurs with M. 

 crispus, M. exiguus and M. Marri. 



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

 the Authors. 



Monograptus discus, Tornquist. Plate XLTV, figs. 5 a — d. 



1883. Monograptus discus, Tornquist, Bergbyggn. inom Siljansomradet, Svensk. Greol. Undersok., 



ser. c, pp. 24, 25. 

 1892. Monograptus discus, Tm-nquist, Siijansoniradets G-rapt., ii, p. 39, pi. iii, figs. 27, 28. 



Polvpary very small, coiled into a close plane spiral so as to appear like a small 



disc ; widening very rapidly from origin to a 

 Figs. 302 a- «.- Monograptus discus, maximum breadth of about l'o mm. Sicula 



Tornquist. 



very long and conspicuous. Thecal twenty 

 4i>.^V' those of M. priodou, but broader in proportion 



: tc4X% i t° sixteen in 10 mm., of the general type of 



^>f f V those of M. priodou, but broader in proportion 



to their length, the hook occupying only one- 

 a /, c third of the total breadth of the polypary. 



a. Young specimen, showing sieuia. Description. — The small disc-like form of the 



At'on Tyn-y-rhos, Talerddig, near . . . 1 . 



Lianbrynmah-; Talerddig Heds. polypary is highly characteristic of this species, 

 6. Somewhat larger specimen ibid. though there is considerable variety in different 



c. Large specimen, showing form of .-,.., , ., n e ,-, • ■, -. 



theca?. Tarannon River. Mont- individuals as respects the ClOSeiieSS Ot tlie COlI, and 



w'ood yshire; GelU lk ' ds ' C " 1L hence the perfection of the disc. The outline of 



some .forms is practically circular, the circle 

 measuring about 4 mm. in diameter, but others are subelliptical and measure 4 

 by mm. 



The sicula is curved, and relatively very long and conspicuous, measuring 

 1 mm. in length and reaching practically to the top of th. 2. 



58 



