Vol. 56.] FORMATION AND ITS GRAPTOLITE-FATTNA. 477 



sicula appears to extend to about the level of the aperture of the 

 second theca. 



Thecae. — Thirty to twenty-four in the inch (twelve to nine in 

 1 cm.), inclined to the axis at an angle of 30° to 35°. Original shape 

 of the thecae somewhat difficult to determine, as it varies considerably 

 in different aspects. In some views the thecal aperture appears to 

 be circular, as in Monograptus vomerinus, and the upper wall is 

 prolonged into a long curved spine (text-fig. 20 b, p. 476). In other 

 views the aperture is not so well seen, the upper thecal wall curving 

 over it as a short claw which is prolonged into a spine directed down- 

 ward, reminding one of M. riccartonensis, Lapw. (text-fig. 20 a). 

 There are also appearances intermediate between these extremes. 

 The various aspects of the thecae are shown in the figures. Lobe- 

 like projections of the thecae occupy only about a quarter of the 

 total width of the polypary. The adult thecae are about 1*27 mm. 

 (•05 inch) long, rather more than '63 mm. (*025 inch) wide, being 

 therefore barely twice as long as wide. Thecae in contact only, or 

 with slight overlap. 



The occurrence of M. uncinatus var. orbatus in the Lower Ludlow 

 Shales is of particular interest, since the variety forms a connecting- 

 link between the graptolitic fauna of the Wenlock and Ludlow. 

 It combines some of the characters of the M.-colonus group, so 

 characteristic of the Ludlow, with some of those of the M.-priodon 

 and M.-vomerinus groups of the Wenlock, for it resembles the former 

 (1) in its general shape, (2) in the character of the proximal 

 extremity, and (3) in the distally-produced virgula, while it is allied 

 to the latter in the shape and form of the thecae. 



It may be readily distinguished from all other Ludlow graptolites, 

 except those of the group of M. uncinatus, by : — 



(1) The generally irregular form of the polypary ; and 



(2) The character of the thecae. 



It is undoubtedly closely allied to M. uncinatus, Tullb., of which 

 it must be regarded as a variety. The English form may be 

 distinguished from the Swedish species by :— 



(1) Its irregular form (M. uncinatus is nearly straight) ; 



(2) The form of the sicula ; 



(3) The larger number of thecae to the inch (thirty as against twenty-four) ; 



(4) The spinose termination of the apertural claw of the theca. 



Localities. — Long Mountain (Trefnant-Middletown Brook). 

 Horizon. — Zone of M. Nilssoni. 



Associates. — M. bohemicus, M. Nilssoni, M. vulgaris var. a, 

 and M. varians. 



Yar. miceopoma (Jaekel). (PI. XXV, figs. 24 a & 24 b and text- 

 fig. 21, p. 478.) 



1889. Tomatograptus micropoma, Jaekel, Zeitschr. d. Deutsch. Geol. Gesellsch. 

 vol. xli, pi. xxix, figs. 4-6. 



I have found only a few British specimens of this form, and these 

 are somewhat indifferently preserved. I have no doubt, however, 

 Q. J. G. S. No. 222. 2 k 



