Vol.65.] HARTFELL-VAEENTIAN SUCCESSION ABOUND PLYNLIMON. 535 



Monogkaptus BHEIDOLENSIS, sp. nov. (Fig. 19 a, b, C, & d.) 



Polypary of unknown length, but probably exceeding 12 inches 

 (30 cm.), gently curved near the proximal end and almost straight 

 distally. Width increasing slowly and uniformly from # 01 inch 

 (•25 mm.) opposite the aperture of the first theca to -08 inch (2 mm.) 

 in the most distal portions. Sicula = *2 inch (5 mm.) in length,- 

 tapering gently from a maximum width of -005 inch (*12 mm.); 

 its apex reaches as far as the origin of the third theca. The 

 thecse are long tubes, exceedingly slender about the middle of 

 their length and expanding towards the apertures ; their outer 

 walls are concave, or bent into a gentle sigmoidal curve. They are 

 disposed on the convex side of the polypary and are "16 inch 

 (4 mm.) long near the proximal end and in the distal portions, 

 but -2 inch (5 mm.) where the curvature of the polypary is 

 greatest ; their width at the aperture is from a tenth to a 

 twentieth part of their length. Their overlap near the sicula is 

 one-half, but after the first few thecse it increases to two-thirds, 

 each theca then originating nearly opposite to, or slightly in 

 advance of, the aperture of the third theca below. They are in- 

 clined at a small angle to the virgula and number about thirteen 

 to the inch (five or six in 10 mm.) near the sicula; but are more 

 closely set distally, where there are about twenty-three to the inch 

 (nine in 10 mm.). 



The apertural margin is simple and nearly normal to the axis of 

 the theca, or in some cases slightly everted. In compressed speci- 

 mens the widening of the thecse towards the aperture is greatly 

 accentuated ; the outer walls are then strongly concave, and the 

 slightly everted apertural margins appear to be prolonged into a 

 distinct lip. 



Horizon and locality. — Monograptus rheidolensis is fairly 

 common in the Pont Erwyd district in the zone which bears its 

 name, and it may also occur in the succeeding zone of M. cyphus. 

 Its commonest associates are Climacograptus tornquisti, Elles & 

 "Wood, and CI. hughesi, Nich. 



The thecse of this species resemble somewhat those of Mono- 

 graptus gregarius, Lapw., but their much greater length and overlap 

 in M. rheidolensis serve to distinguish the two species even in 

 fragments. The only species with which M. rheidolensis might be 

 confounded is M. leptotheca, Lapw., which it resembles in the great 

 length and overlap of the thecee ; but the greater curvature of 

 the polypary and the details of the thecal structure sufficiently 

 distinguish them. 



The figured specimens are deposited in the Museum of Practical 

 Geology, Jermyn Street, London, the numbers appended to the 

 text-figures being the register-numbers of the specimens from which 

 they were drawn. 



My sincere thanks are tendered to Miss E. G. Welch, who 

 prepared the drawings of the graptolites. 



