"Vol. 56.] FORMATION AND ITS GRAPTOLIT E-FATJtfA. 463 



(b) Group 2. Type Monogbaptus colonus (Barr.). 



1. Two types of thecae are present, the proximal thecae possessing 

 recurved apertures. 



2. The outer wall of the first theca arises above the aperture of 

 the sicula, and is inclined to it at a considerable angle (40° to 45°). 



3. The length of the adult theca is, as a rule, 4 or more times 

 the width. 



Different species belonging to this group vary considerably in the 

 form of the thecae, but in all, the first two characteristics noted 

 above are invariable. 



Monogbaptus colonus (Barr.). (PI. XXV, figs. 10 a-10 d and 

 text-fig. 14, p. 464.) 



1850. Graptolitlms colonus r Barrande, ' Grapt. de Boheme ' p. 42 & pi. ii, figs. 2-3. 



There are probably few species that have been so frequently 

 quoted by graptolithologists as this form, and few whose identifi- 

 cation has been so uncertain. This is largely due to the fact that 

 Barrande, under the name of colonus, figured three obviously 

 different species {op. Git. pi. ii, figs. 1-5). Even when it became 

 the general practice to regard Barrande's figs. 2 & 3 as the type- 

 specimen, the difficulties of identification were by no means all 

 removed, for doubt was still felt as to the accuracy of the figure. 

 We now know that such doubts were well founded, for the artist's 

 figure is not a reproduction of a complete specimen, but rather an 

 inaccurate restoration in which the thecae are drawn as if they were 

 all of the form of those at the proximal extremity. 



Fortunately there is now no room for doubt a& to the true 

 characters and form, not only of the proximal, but also of the distal 

 thecae of Barrande r s original specimens on which he founded this 

 species. Dr. Perner has refigured and redescribed these specimen?,. 

 and through his kindness I was enabled to examine and draw 

 them for myself. The following description, however, is drawn 

 mainly from English specimens from the Lake District and from 

 Builth, supplemented, where necessary,, from these Bohemian forms.. 



Polypary. — 3*8 to 5*8 cm. (1*5 to 2 inches) long, occasionally 

 exceeding the latter limit. Straight distally, but possessing a 

 distinct dorsal curvature in the proximal 6'3 mm.. (-25 inch) or so 

 of its length. Width at the proximal end = about -84 mm. (-033 

 inch), exclusive of the apertural spine. Increase in width at the 

 rate of about "127 mm. (*005 inch) for each theca for the first six 

 or eight thecae, then much more gradual, and for the distal third 

 or half of the length of the polypary the width is uniform. 

 Maximum width = 2*3 mm. (-09 inch). Yirgula slightly pro- 

 longed distally, rarely seen. 



Proximal Extremity. — Sicula generally conspicuous, rather 

 less than 1-9 mm. (-077 inch) long, and about -32 mm. (-012 

 inch) wide at the aperture, so that it is 6 times as long as wide. 

 Aperture provided with a long slender ventral spine. First theca 



2i2 



