442 



BRITISH GRAPT0LITL3. 



Figs. 304 a-c. — Monograpttts Sedg- 

 wickii ( Portlock). 







with the majority of examples known from elsewhere. Proximal fragments, 

 however, wherever they are met with, are distinctly flexed in all cases and have 

 a general limp appearance. • 



The sicnla in this species is relatively small, measuring about 1'4 mm. in 

 length. 



All the earliest thecae are small, with distinctly retroverted apertnral regions 

 occupying two-thirds of the breadth of the poly pary and strongly recalling those of 

 M. priodon. As the thecas successively increase in size, however, the proportion of 



the retroverted part of the apertnral region presents 

 the appearance of correspondingly diminishing in 

 amount, until finally only the plane of the apertnral 

 margin itself faces the proximal end of the polypary 

 and occupies only one-third of its total breadth. 

 This apertural margin is not infrequently thickened 

 in the larger and more distal thecae, and this 

 thickening is often produced outward as a distinct 

 and conspicuous spine. In the most distal thecas 

 there is comparatively little retroversion shown in 

 the apparent ventral edge, while the spine has be- 

 come the conspicuous feature of the theca (Fig. 

 304 d). In the proximal part of the polypary the 

 spine is, as a rule, insignificant even in well-preserved 

 specimens, although the aspect be such as to place it 

 in such a position that its entire length is presented 

 to the observer. There is one aspect of the polypary 

 (reverse) in which, as the result of the torsion of 

 the thecal axis, the spine may be more or less 

 completely hidden from the observer behind the 

 iaiKd Waie? eo1 s_urvey of ' En8 '~ intervening membrane of the thecal wall, or 



embedded in the rock matrix ; and even in the case 

 of the obverse aspect the spine may be non-apparent for much or the whole of its 

 length, being pressed down upon the upper surface of the fossil. But the variations 

 noted above in the appearances presented by the thecas have all been observed in one 

 and the same individual in several different specimens (compare Figs. 304 (/ and e). 

 Spines are certainly present on Portlock's examples, though all specimens 

 obtained from his typical locality of Limehill are so poorly preserved that the detec- 

 tion of the spines is a matter of some difficulty. In specimens from the Lake 

 District, on the other hand, the spines are often very beautifully seen, extending some 

 2 — 3 mm. beyond the thecal margins. It was to these conspicuously spined Lake 

 District examples that Nicholson applied the name var. sjpinigerus. Since, how- 

 ever, the Irish specimens have spines also and the perfection in which they are 



a. Proximal end, showing sicula and 



form of proximal thecal Enlarge- 

 ment of part of PI. XLIV, fig. 10 c. 



b. Slightly more distal thecae, showing 



form of apertural margin and be- 

 ginning of spine. Plew lands Burn, 

 Raehills, S. Scotland : Birkhill 

 Shales (Zone of Monog. Sedg- 

 wickii). Geol. Survey of Scotland. 



c. More distal theca? with longer spines. 



Parys Mountain, Anglesea ; Llan 



