C. Lapworth — On Scottish MonograptidcB, 355 



is isolated (Plate XIII. Fig. 1 d). This isolation is not apparent in 

 flattened specimens, but the thecas are pressed into complete contact 

 with each other throughout. The margin of the thecal aperture is 

 broadly convex, and there is usually a very distinct and somewhat 

 mammilate denticle. In some specimens the latter is far more acute, 

 and is prolonged to a distance equal to half the transverse diameter 

 of the polypary. It is almost invariably furnished with a stiff 

 horizontal spine, very conspicuous. The convexity of the apertural 

 margin is not so apparent in examples which are in a state of half- 

 relief, and it frequently happens that the broad aperture itself is 

 directly visible, having been compressed from above. 



The distant, broad hydrothecge, with their peculiarly shaped 

 apertural margin, and stout horizontal spine, at once separate this 

 species from all its associates. Its nearest ally is M. Biccartonensis, 

 Lapw. 



Locality, — Monograptws Halli is confined to the highest zones of the 

 Birkhill Shales at Garple, Frenchland, Mountbenger, Dobbs Linn, 

 Craigmichan, Glenkiln, Duffkinnel, etc. 



19. MoNOGRAPTUS EiccARTONENSis, sp. nov. Plate XIII. 

 Figs. 2 a — e.. 



Polypary straight, or with a very slight dorsal curvature, and 

 having a general diameter of one-twelfth of an inch. Hydrothecae 

 26 to 2:8 to the in<;h, inclined at an angle of about 50° overlapping 

 each other for a small fraction oi their length; aperture wide, with 

 a broadly convex naargin ; denticle long, everted and produced in a 

 stout spine. 



The proximal end of the polypary is usually bent sharply back- 

 wards, and exhibits a short and broad sicula. The hydrothecse are 

 very similar in form to those of M. Halli ;. and the thecal aperture 

 shows the same sudden distal expansion and convex lateral margin. 

 In almost every case the appearance presented by the compressed 

 polypary is that given in Plate XIII. Fig. 2e, where it is compressed 

 partly laterally, partly ventrally, and the whole extent of the thecal 

 orifice, very slightly distorted, is presented to the observer. The 

 lateral angle of the aperture clearly forms the denticle visible on the 

 ventral margin, and gives origin to a stout spine, which is directed 

 partly outwards^ and partly towards the distal extremity of the poly- 

 pary. As the same appearance is presented when the polypary is 

 compressed, so that the polypiferous margin lies either to the right or 

 to the left of the fossil, it is evident that the aperture must have been 

 furnished with two spines, one on each of the lateral angles. This 

 is probably the case also in the remaining spinose Monograptidoe, but 

 the evidence is not usually so conclusive. 



On a cursory examination, it appears clearly evident that it is the 

 prolongation of that portion of the polypary beyond, or on the distal 

 side of the aperture, that gives origin to the recurved ventral spine, 

 and in poorly-preserved examples it is very difficult to resist the 

 impression that it is given off from the base of the following hydro- 

 theca. 



