C. Lapworth — On Scottish Monograptidce. 353 



augmentation in diameter throughout the whole of its extent, which, 

 judging from the fragments known, must have exceeded 18 inches. 

 The curvature in the earlier moiety is very slight, but sufficient to 

 enable us to perceive that the thecae were arranged on the convex 

 margin of the periderm. 



The first-formed hydrothecee, though almost identical in shape 

 with those on the distal portion of the polypary, only overlap each 

 other for about half their extent. As the length increases, this over- 

 lap rapidly augments, until, in the later and fully developed portion 

 of the polypary, the hydrothecae become remarkably slender flattened 

 tubes, one-tenth of an inch in length, lying almost parallel with 

 the line of the virgula. Only one-fourth of the length of each is 

 free, the remainder being overlapped by the preceding theca. Each 

 originates opposite the aperture of the third preceding it ; so that a 

 transverse section of the polypary crosses three hydrothecae. The 

 free portion is a little sacculate ; the exterior margin lies parallel 

 with the virgula ; and the outer orifice is placed partly within the 

 line of the ventral edge of the polypary. The interspace is very 

 short and the apertural margin shallow, and either horizontal, or, in 

 specimens in relief, a little acute. The denticle is generally some- 

 what mucronate, but is otherwise destitute of ornament. 



From all other forms of the group, M. leptotheca is at once distin- 

 guished by the form and length of its distal hydrothecse. 



Locality. — Eare in the middle zones of the Birkhill Shales at 

 Dobbs Linn and Frenchland Burn. 



17. MoNOGRAPTUS voMERiNUs, Nich. Plate XTI. Fig. 6 a — e. 



GraptoUthus colonus, Barrande ; Grapt. de Boheme, pi. ii. fig. 4 ; 



Suess, Bohmische Graptolithen, Taf. viii. a—f. 

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



xxiv. pi. XX. figs. 7, 8. 

 Graptolites vomerinus, Nicholson ; Monogr. British Graptolitidae, 

 p. 53, fig. 21. 

 Polypary straight, rigid, four or five inches in length, and rapidly 

 increasing in diameter to a maximum width of one-tenth of an inch. 

 Hydrothecse 20 to 22 to the inch, inclined at an angle of from 30^ to 

 40"^ ; short and broad cups, in contact for half their extent ; distal 

 portion of outer margin perpendicular, proximal portion sharply 

 impressed — forming a deep excavation, and bearing a short horizontal 

 mesial spine : apertural edge short, concave and normally devoid of 

 ornament. 



The polypary in this species is a straight, stifi" rod, several inches 

 in length, and with a test of remarkable thickness. The proximal 

 end is abrupt, and the sicula short and broad. When preserved in 

 relief, the hydrothecae may be likened to those of Monogr. priodon, 

 Bronn., truncated immediately at the point where they begin to 

 isolate themselves. But the species to which it is most intimately 

 allied, is M. Hisingeri, Carr. There is the same absence of over- 

 lap and the same short cup-like hydrothecse. The latter, however, 

 differ most materially in the details of their form. In the present 



DECADE II. — VOL. III. — NO. YIII. 23 



