C Lapworth — On Scottish Monograptidce. 505 



edge is undistinguishable in the fossil from the body of the hydro- 

 thecas, upon which it is crushed down; while the long rounded 

 margin which is the more distant from the observer, forms the 

 visible distal boundary of the theca. In specimens preserved in 

 strict profile, or with their full relief, the nearer margin of the 

 aperture is visible and the obliquity is always much less. The 

 proximal boundary of the denticle is formed by the expanded part 

 of the theca immediately preceding the aperture. 



According to Mr. Salter the orifice is situate on the lower or 

 inferior side of the denticle, and the latter "overhangs the wide con- 

 cave margin of the mouth, which abuts at its lower edge directly 

 against the succeeding cell." In this view he was strengthened by 

 Barrande's reading of the characters of his M. chimera, in which the 

 relations of theca and denticle are similarly interpreted. A glance 

 at Mr. Salter's figures is sufficient to prove that this view is wholly 

 erroneous. 



II. Flemingii is intimately allied to M. Halli and if. Hiccartonensis. 

 From the former it difi'ers in the number and character of the 

 hydrothecED, and from the latter in the form and size of the theca and 

 polypary. 



Locality. — Common in the Eiccarton beds at Kiccarton, Shanken- 

 shiels, Balmae, etc. 



34. MoNOGRAPTUs ooLONUs, Barrandc. Plate XX. Fig. 9. 



Graptolithus colonus, Barrande ; Grapt. de Boheme, pi. ii. figs. 

 1, 2, 3. Geinitz, Die Graptolithen, Taf. ii. figs. 33 to 36. 

 Nicholson, Quart. Journ. Geol. Soc, vol. xxiv. pi. xx. figs. 

 9, 10, 11. 



Polypary rarely exceeding a couple of inches in length, straight, 

 rigid ; rapidly expanding to a maximum width of one-tenth of an 

 inch. Yirgula distally prolonged. Hydrothecse 24 to 28 to the 

 inch, inclined at an angle of about 30°, contracting slightly in the 

 direction of the aperture, the margin of which is oblique, concave 

 and ornamented with a minute spine. 



This species is remarkable among those of the Halli group in the 

 shortness of the polypary and in the constancy with which the 

 virgula is distally prolonged. The length of this extension varies 

 from one-half to more than the whole of that of the polypary itself, 

 and communicates a very distinctive aspect to the species. 



The dorsal margin of the polypary is straight, except at its proxi- 

 mal end, where the small portion carrying the sicula and the first 

 four or five hydrothecse is usually bent a little inwards. The sicula 

 itself is always very conspicuous, and the polypary springs out 

 almost immediately to its full diameter. 



The hydrothecaa differ slightly in appearance according to the 

 variation in the direction and amount of compression. Each is a 

 narrow tube, of which the length is about five times the width — with 

 lateral margins either straight, or having the slight and elegant double 

 curvature of the majority of the species of the Ilalli group. There is 

 an apparent diminution in width in the direction of the aperture, which 



