454 



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



purposes of separating the species belonging to the groups of Mono- 

 yraptus dubius and M. colonus. The relative width and length 

 of the thecge vary considerably in different species, and are in some 

 cases the same in the one group as in the other. It is impossible to 

 identify a species belonging to either of these groups without a 

 careful examination of the proximal end of its polypary. 



MONOGRAPTUS DUBIUS (Suess). (PI. XXV, flgS. 1 A & 1 D.) 



1851. Graptolithus dubius, Suess, ' Ueber Bohmische Graptolithen ' Haidinger's 

 Abhandl. vol. iv, pt. iv, p. 115 & pi. ix, figs. 5 a-b. 



(Note. — The synonymy of the several species will be found on p. 487.) 



This is typically a Wenlock form, but as it ranges up into the 

 Ludlow Beds I here give a brief description of it, drawn from 

 specimens found in the M.-Nilssoni zone of the Montgomery-road 

 section and in the Ludlow district. 



Polypary. — Attains a length of 10 cm. (4 inches). Straight 

 for the greater part of its length, but for the first five or six thecas 

 slightly curved inward. Width at proximal end = about *76 mm. 

 (03 inch); maximum width = only 2 mm. (*08 inch) ; increase in 

 width gradual throughout the polypary, but rather more rapid for 

 the first few thecae. Distal prolongation of the virgula rarely seen, 

 owing to the great length of the polypary. 



Proximal Extremity. — Sicula 1*77 mm. ('07 inch) long 

 and '42 mm. (-017 inch) in diameter at the apertures, hence 

 4| times as long as wide. Aperture con- 

 cave and slightly contracted, with a short 

 curved ventral spine. Apex of the sicula 

 extending to just below the aperture of 

 the second theca, and the outer wall of the 



Fig. 0. — M 



dubius, 

 proximal extremity 



showing the sicula. 



(x5.) 



first theca reaching to the base of tb< 



sicula and inclined to it at a low angle 

 of 20°. 



Thecae. — Twenty -five to twenty in the 

 inch (ten to eight in 1 cm.), inclined to 

 the axis at an angle of 30° to 35°. All 

 of the same type ; fairly long and broad 

 tubes, 2 or 3 times as long as wide, and 

 overlapping for from a third to a quarter 

 of their length. Length of an adult theca 

 = 2-54 mm. (-1 inch). Thecal apertures 

 round or slightly oval, provided with a 

 blunt denticle, generally ornamented with 

 a small spine. 



M. dubius is readily recognized by the 

 [Enlargement of fig. 1 a shape and number of its thecas, but it often 

 in PL XXV.] varies considerably in the general form of the 



polypary, At some horizons in the Wenlock Shales it rarely exceeds 

 2-5-1 to 3*8 cm. (1 to 1-5 inches) in length, and has a long distal 

 prolongation of the virgula ; at others again, it reaches a length of 



