C. Lapicorth — On Scottish Monograptidce. 319 



diameter. The hydrotliecse are narrow and linear ; their outer 

 margins, instead of being tolerably straight, as in the majority of the 

 species of this group, are very elegantly curved. The distal moiety 

 of each is convex and sub-parallel with the axis of the polypary, 

 while the proximal portion is sharply impressed. In this way a 

 slight excavation is formed, into which opens, in part, the external 

 orifice of the hydrotheca immediately preceding; thus reminding 

 us of the DicranograptidcB. The apertural margin is slightly concave, 

 and often greatly inclined, so that the denticle is acutely triangular. 



This species is unique in this group in the form and habit of the 

 hydrothecse. From M. cyphus, the only species with which it can 

 possibly be confounded, it is separated by the form of the hydrothecse 

 and their want of overlap. 



Locality. — Eare in the upper zones of the Birkhill beds at Dobbs 

 Linn. 



11. MoNOGRAPTUS TENUIS, Portlock sp. Plate XL Figs. 3 a—n. 

 Graptolites tenuis, Portlock ; Geol. Eep., Tyrone, pi. xix. fig. 31. 

 Graptolites discretus, Nicholson ; Quart. Journ. Geol. Soc. vol. 

 xxiv. plate xx. figs. 12, 13, 14. 



Polypary four to six inches in length; distal portion almost 

 straight, and with a maximum diameter of one-twentieth of an inch ; 

 narrowing rapidly in the proximal portion, which is broadly recurved. 

 Hydrothecee linear, adnate to the convex margin of the periderm, 16 

 to the inch ; outer margin straight, apertural edge direct ; denticle 

 obscure, or (in distal portion) long and inclined, or even (in proximal 

 portion) strongly reverted. 



This is one of the most variable species of the Monograpti. In 

 the greater number of specimens the adult portion of the polypary 

 is provided with closely-set hydrothecae with straight boundaries, and 

 widening gradually in the direction of the aperture, the margin 

 of which transgresses a little upon the body of the periderm, and 

 is either normal to the general axis or but slightly inclined. Oc- 

 casionally, however (and this occurs almost invariably in the more 

 recent examples), the denticle is long, acuminate, and highly inclined. 



In the proximal portion the hydrothecse are far more distant. In 

 many examples they closely resemble the thecae of the adult portion, 

 and possess similar short obscure denticles. In others the latter 

 are much pointed and prolonged ; and frequently the distal extremity 

 of the theca is bent completely round, so as to face the proximal 

 extremity of the polypary. (Plate XL Fig. Z g. Compare also 

 Nicholson, loc. cit. sup., fig. 14.) 



The entire fossil has a dorsal curvature, but the distal portion is 

 comparatively straight. The proximal division is bent normally into 

 a broad arch. Frequently the polypary is very narrow at its origin, 

 and the augmentation in diameter is so rapid that the maximum 

 width is reached within the first three inches of length ; but oc- 

 casionally the proximal portion is slender, flexuous and of remark- 

 able length — the whole polypary widening imperceptibly throughout. 



M. tenuis is effectually distinguished from all its congeners by the 

 general form of the polypary and the characters of the hydrothecse. 



