MONOGRAPTUS. 415 



Distally the polypary may appear to be nearly straight, but it commonly shows 

 curvature to some slight extent ; in the proximal portion the curvature is, as a 

 rule, distinctly dorsal. 



The mature thecse are similar in form to those of M. vomerinus, but 

 possibly with rather less torsion of the thecal axes. The apparent narrowing of 

 the thecas in the apertural region may be real, but it is at any rate exaggerated by 

 the torsion, and may be entirely induced by it. Occasionally, as the result of 

 compression, the twisted apertural region shows a tendency to isolation, and an 

 effect almost of retroversion is produced ; it is evident, however, from the study of 

 specimens preserved in different ways that this appearance is produced entirely by 

 the conditions of preservation. 



Affinities. — The slender flexed character of the polypary serves to distinguish 

 Manor/, crenularis from all allied species other than M. griestoniensis, from which 

 it may be readily separated by its lesser curvature and greater breadth. 



Horizon and Locality. — Llandovery (Birkhill Shales), zone of M. convuluhis. 



S. Scotland. — Dobb's Linn, etc. Central Wales: R. Twymyn, above Llan- 

 brynmair. 



Associates, etc. — Monog. crenularis occurs in S. Scotland associated with Gephalog. 

 conieta, Petalog. folium, Glyptog. tamariscus var. incertus, Orthog. bellulus, Monog. 

 lobiferus, M. convolutus, M. Clingani, M. denticulatus, etc. It is not a very common 

 fossil. 



Collections. — Geological Survey of Scotland and Lapworth. 



Monograptus galaensis, Lapworth. Plate XLII, figs. 1 a — c. 



1876. Monograptus galaensis, Lapworth, G-eol. Mag. [2], vol. iii, p. 356, pi. xii, figs. 5 a — d. 



Polypary several cm. in length, straight or very slightly flexed, widening fairly 



rapidly from an initial breadth of "6 mm. to a maximum of about 2 mm. 



Thecse ten to nine in 10 mm., with slight ogee curvature, and contracting 



somewhat in the apertural region ; those of the proximal portion with a 



tendency to retroversion, those of the distal portion everted, with the 



apertural margin thickened and produced into a distinct denticle or tag. 



Overlap one-third to two-thirds of the total thecal length. 



Description . — The polypary is quite slender in the proximal portion, wdiich is 



slightly inflexed in the immediate neighbourhood of the sicula; it, however, attains 



its maximum breadth fairly quickly and is then uniformly wide for the greater 



part of its extent. The maximum width of 2 mm. is, however, rarely attained, 



a breadth of 1*5 mm. being of commoner occurrence. 



The sicula has a length of 1 mm., and the theca? of the proximal portion are 

 relatively broad w r ith a decided tendency tow r ards retroversion of the apertural 



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