MONOGRAPTUS. 



407 



Figs. 273 a—d. — MonoQraptiis tenuis (Portloek). 



fv 



Monograptus tenuis (Portloek). Plate XL, figs. 2 a — e. 



1843. Graptolites tenuis, Portloek, Geol. of Londonderry, y>. 319, pi. xix, fig. 7. 



1868. Graptolites discretus, Nicholson, Quart. Journ. Geol. Soc, vol. x.xiv, p. 539, pi. xx, figs. 12 — 14. 



1876. Monograptus tenuis, Lapworth, Geol. Mag., dec. ii, vol. iii, p. 319, pi. xi, figs. 3 /(, A-, /, m, n. 



Polypai'j of great length, slightly arcuate throughout, widening gradually from 

 an exceedingly slender proximal portion till the maximum breadth of 1 mm. 

 is attained. Thecte seven to five in 10 mm., long and narrow, with distinct 

 sigmoid curvature, overlapping one-third to one-half their length; those of the 

 proximal portion with abrupt expansion in the apertural region and usually 

 introverted, those of the distal portion even and slightly introverted. 

 Bescription. — The polypary must have had a length of fully 10—15 cm., and 



though in the extreme proximal portion it 

 appears to widen very gradually, it attains 

 its maximum breadth at a comparatively 

 early stage, usually within the first 4 or 

 5 cm. of its length. 



The tliecfB are long and slender; they 

 show a sigmoid curvature of their walls as in 

 the genus Lejytograptas, but the apertural 

 margin, while always tending to be intro- 

 verted, is not always conspicuously so, owing 

 to the effects of compression upon the thecfB, 

 which expand abruptly in the apertural 

 region. In the proximal portion this may 

 give rise to an appearance of retroversion 

 of the apertural margin in some cases (though 

 in other cases it is clearly introverted), and 

 in the case of the mature thecse of the distal 

 portion may tend to the production of an 

 acute denticle. The thecse are distant and overlap each other barely one-half 

 their length. 



Afjhiities. — For a considerable time Monog. tenuis, Portloek, and M. discretus, 

 Nicholson, have been regarded by some as different species, but an examination of 

 Portlock's Irish specimens and new material collected from the type locality shows 

 that the two are identical. A distinct form which occurs at lower horizons, and 

 which has been erroneously referred to M. tenuis by various authors, has recently 

 been distinguished and described by Jones as M. atavus. 



Monog. tenuis differs from M. atavus mainly in the characters of the thecte, 

 which are more elaborate than those of M. atavus, in which there is no sudden 



54 



I y 

 b 



I 



; 



C (I 



Enlargement of part of 



a. Proximal portion 



PI. XL, fig. 2 c. 



b. Distal theca! in ordinary profile view. Enlarge- 



ment of specimen on same slab as fig. 273 a. 



c. Thecoe not in ordinary profile view, giving 



appearance of thecal spine. Ihid. 



d. Distal thecse. Enlargement of part of PI. XL, 



fig. 2rt. 



