MONOGRAPTUS. 



IP.) 



Pigs. 282 a, b. — Monograptus priodon 

 (Broun). 



Polypary of great length, straight and rigid, widening gradually and persistently 

 from an initial breadth of *75 mm. to a maximum of 3 mm. Sicula con- 

 spicuous. Thecae thirteen to eight in 10 nun., showing pronounced ogee 

 curvature, narrowing towards the apertural region, with considerable torsion 

 of the axis and with folly two-thirds of the total thecal length involved in 

 the prominent hook, which occupies fully one-half of the total breadth of 

 the polypary. Overlap one-third to two-thirds the total length. 

 Description. — The polypary of Monog. priodon must have attained a very great 

 length, since fragments 10 — 12 cm. in extent are of fairly frequent occurrence. 

 Those in beds of Gala-Tarannon age are usually shorter than those from the 

 Wenlock Shales, rarely attain a breadth exceeding 2*5 mm., and commonly 



average about 2 mm. only. The specimens which 

 come from the Wenlock Shales are, as a rule, longer 

 and wider, measuring fully 3 mm. in breadth. 



The proximal end is relatively slender, not 

 exceeding 1 mm. in width ; and the dorsal wall of 

 the polypary is continuously straight, though the 

 position of the sicula sometimes causes an appear- 

 ance of slight retroflexion; the sicula itself has a 

 length of 1'5 mm. and reaches nearly to the top of 

 th. 2. 



The thecas in the proximal portion of the poly- 

 pary are all fairly small and measure thirteen in 1 

 mm. ; in the distal portion, however, they are larger, 

 and measure eight in the same unit of length, 

 Proximally about one-half of the theca only is 

 involved in the hook, which occupies about one-half 

 of the total breadth of the polypary ; but distally 

 fully two-thirds is involved, though the hook occupies but two-fifths of the total 

 breadth ; it is clear therefore that in the maturer part of the polypary the hook 

 portion is relatively longer. The plane of the thecal aperture in all cases faces the 

 proximal end of the polypary, making approximately an angle of 135° with the 

 axis of the polypary itself. In some specimens there is an appearance of expan- 

 sion near the apertural margin, but this is not noticeable in others. Perner has 

 separated such forms as distinct varieties, but the apparent expansions seem to us 

 to be consequent upon the torsion of the thecal axis in the apertural region, so 

 that after compression the actual aperture is in one aspect (the obverse) facing the 

 observer and in the other turned away. An occasional appearance of a slight 

 obliquity in the apertural margin would seem to be due to the same cause. The 

 thecse as a whole present two extreme but common views according to whether the 

 obverse or reverse aspect is presented. In the reverse aspect (fig. 282 c) the 



a. Proximal end, showing part of sicula ; 



preserved in limestone, in full relief. 

 Builth (?) ; Wenlock Beds. Geol. 

 Survey of England and Wales. 



b. Smaller specimen, in partial relief, 



showing complete sicula. Pen- 

 whapple Glen, Girvan ; Gala beds. 

 Coll. Lapworth. 



