464 



MISS E. M. E. WOOD ON THE LOWEE LUDLOW [May I9OO, 



Fig. 14.— Monograptus 

 colonus (Barr.J. 



H 



arising slightly above the aperture of the sicula, being inclined to 



it at an angle of abont 45°. Sicula 

 extending to the level of the second 

 thecal aperture. 



Thecae. — Thirty-two to twenty-six 

 in the inch (twelve to ten in 1 cm. 1 ), 

 inclined to the axis at angles varying 

 from 35° to 45°, generally about 40°. 

 They are of two distinct types : the 

 proximal four or five thecaa have their 

 apertures strongly recurved, and 

 possess a distinct spine which is bent 

 downward. The succeeding thecaB have 

 no such distinct spines, the apertures 

 being concave, at right angles to the 

 direction of the theca, and possessing 

 only a somewhat pointed denticle to 

 the lower apertural margin. According 

 to Dr. Pernor, in some Bohemian spe- 

 cimens preserved in full relief, the 

 apertures are slightly convex and 

 recurved in some of the more distal 

 thecoe, but this character is not ob- 

 served in ordinary compressed spe- 

 cimens preserved in shale. Proximal 

 thecse only 2| to 3 times as long as 

 wide, the proportion gradually increasing until the adult thecge 

 are 4 times 2 (or even more) as long as wide. The amount of 

 overlap varies from one quarter in the proximal thecae to a third 

 or a half in the adult thecoa. The length of an adult theca 

 exceeds 2*6 mm. ('1 inch). 



M. colonus has been almost universally quoted correctly as 

 marking the horizon of the Lower Ludlow Shales in Britain and 

 Europe. The species, however, as now defined, is by no means 

 so common, in Britain at any rate, as has been generally supposed. 

 In the Lower Ludlow Shales of Ludlow itself M. colonus is rare, 

 and the only specimens that can be identified with it are those 

 figured by Prof. Lap worth as M. Roemeri : these are perhaps hardly 

 typical, as I shall subsequently point out. Where the mudstone- 

 facies of the Lower Ludlow is developed, as at Builth, M. colonus 

 occurs in great abundance, and is one of the most characteristic 

 graptolites of the M.-Nilssoni zone. 



Although the foregoing description is significant of the typical 

 form of M. colonus as found in Bohemia, yet in each British 

 locality where this species occurs it is met with under certain 

 slight variations ; these local variations I do not, however, consider 



a — proximal extremity, show- 

 ing the sicula and the 

 form of the proximal 

 theca ; from Vicarage 

 Road, Builth. (x5.) 



£ = Distal thecrc of fig. 10 b 

 in PI. XXV, partly 

 restored. (X5.) 



1 Dr. Perner reckons ten to eight in 1 cm., but this is undoubtedly a misprint. 



2 See Perner, ' Etudes sur les Graptolites de Boheme ' pt. hi, sect, b (1899) 

 pi. xiv, fig. 3. 



