Vol. 65.] HAKTEELL-VALENTIAN SUCCESSION ABOUND PLTNLIMON. 533 



seen ; compressed specimens are usually somewhat wider. The 

 sicula ="05 inch (1-25 mm.) in length, its apex reaching to about 

 the middle of the first theca. The theca? are short tubes, five or 

 six times as long as wide, of almost circular section, and inclined 

 at a small angle to the direction of the virgula. In the proximal 

 portion they are '06 inch (1*5 mm.) long, and overlap to about 

 a quarter of their length ; distally they increase to *1 inch (2'5 mm.) 

 and overlap to one-half. Their outer wall usually is strongly 

 sinuous (see especially fig. 18 b) and there is sometimes a distinct 

 excavation opposite the aperture of the theca next below. They 

 number about twenty-three to the inch (nine in 10 mm.) along 

 the whole length of the polypary, and in specimens in relief they 

 are usually marked with conspicuous lines of growth. The apertural 

 margins are simple, fairly straight, and lie perpendicular to the 

 length of the thecse. 



Horizon and locality. — This graptolite occurs abundantly in 

 the Pont Erwyd district, in the zones of Monograptus atavus, 

 M. cyphus, in the lower part of the zone of M. communis, and 

 less commonly in the zone of M. rheidolensis ; its associates are the 

 usual graptolites of those zones. 



This form has commonly been referred to Monograptus tenuis 

 (Portlock) ; but, according to Miss G. L. Elles, that species is 

 characteristic of a much higher horizon, and, as noted by Prof. 

 Charles Lapworth, is the same as the one subsequently described as 

 Graptolites discretus by Nicholson. 1 



It agrees with M. atavus in the general form of the polypary, 

 but differs in all its essential characters. In compressed specimens 

 the chief differences are that M. tenuis (Portl.) = M. discretus 

 (Nich.) is a smaller form and has more distant theeae, the apertural 

 margins of which are concave, inclined and bear very acute 

 denticles sometimes prolonged into distinct ' tags.' Specimens of 

 the latter found in relief in the Pont Erwyd district show, however, 

 that the two species are radically distinct. 



The species most nearly allied to M. atavus is M. argutus, Lapw., 

 with which it shows close affinities. The characters of the proximal 

 end are almost identical, but M. argutus is usually more strongly 

 curved. In the distal part the chief distinctions are the smaller 

 size and the more highly developed thecal structure of M. argutus ; 

 the sinuosity and excavation of the theca? is more marked, their 

 apertural margins are inclined, slightly concave, and prolonged 

 into an acute denticle. 



It is probable that the form recorded by various authors as 

 M. tenuis from the Lower Birkhill rocks or their equivalents is to 

 be referred to M. atavus, which appears to be confined to those 

 rocks. 



1 H. A. Nicholson, ' On the Graptolites of the Coniston Flags, &c.' Quart. 

 Journ. G-eol. Soc. vol. xxiv (1868) p. 539 & pi. xx. figs. 12-15. 



