MONOGRAPTUS. t75 



is the one in which the barbed apertural region is entirely concealed and the thecae 

 present a superficial resemblance to those of M. jimbriatus. 



Affinities. — Distal fragments of Mono;/, denticulatus may resemble those of l\l. 

 communis or M. jimbriatus ', but can be distinguished, as the thecseof M. communis are 

 really more conspicuously lobed in their apertural regions, and those of M. jimbriatus 

 far more closely set. If the whole polypary is seen, it is easily separable from 

 those of other species, the two types of thecas rendering it sufficiently distinct. 



Horizon (iiid Localities. — Llandovery (zone of M. con coin fas.) 



S. Scotland: Corrie, Dobb's Linn; Long Linn, Dobb's Linn; Belcraig Burn. 

 Lake District: Skelgill. 



Associates, etc.—Monog. denticulatus is a very common fossil in S. Scotland in 

 the zone of M. convohitus, where it occurs in swarms associated with M. limatulus, 

 M. Glingani, M. lobiferus, Petalog. folium, etc. 



Collections. — Geological Survey of Scotland, British Museum (Natural History), 

 Lap worth, and the authors. 



VI. B. (a) : Monograpti in which the thecas are uniform and the polypary has 



spiral curvature. 



Monograptus spiralis (Geinitz). Plate XLVIII, figs. 7 a—d. 



1842. GraptoUthns spiralis, Geinitz, Neues Jahrb. f. Min., p. 700, pi. x. 

 1850. Grapiolithtis sjnralis, Barrande, Grapt. de Bolithne, p. 54, pi. iii, figs. 10 — 13. 

 1892. Monograptus spiralis /3. subconicus, Tornquist, Siljansomradets Grapt., ii, p. 35, pi. iii, figs. 

 12—23. 



Polypary robust, coiled into an irregular spiral, widening from a slender 



proximal portion persistently throughout its 



Fig. 331 a. — Monograptus spiralis , , rru . ■■ • i , 



(Geinitz). extent. Ihecse on the convex margin, eight 



to nine in 10 mm., of uniform type; sub- 

 triangular with slight torsion, in contact 

 only, free one-fourth to one-fifth of length 

 and with barbed apertural region in which 

 one-half of the free part is involved. 

 Description. — The polypary varies considerably 

 in appearance according to preservation; sometimes 

 it is coiled into a perfectly regular plane spiral (PI. 

 XLVIII, fig. 7 d), at others the spiral is most 



a. Proximal portion, preserved as a cast. . . . 



Thiristane Score, s. Scotland ; band irregular and the convolutions far from symmetrical 



of Rastrites maximus. Coll. Lap- ,,-,, \ ... 



worth. (11. aLVIU, fig. ( ti). Distal fragments which are 



