DIPLOGRAPTUS. 249 



Var. incertus, var. nov. Plate XXX, figs. 9 a — d. 



1876. Diplograptm Huijhesli, Lapworth, Cat. West. Scott. Foss., pi. ii, fig. 37. 



A variety of Glyptog. tamarisQiis, which differs from the typical form in being 

 wider and shorter, and showing a greater amount of overlap of its thecse in the 



bi-profile view, is of common occurrence in the 



Figs. 168 a and h. — Ghndoaraptus n • l l -n en 1 • n • ii £ nr 



tamariscus, var. incertus, nov. Birkhill Shales, especially in the zone oi Monog. 



Sedgwichii. It widens rapidly in the proximal 

 ^ region from *8 mm. to 2 mm., and thence the 



1 ? ■ margins become subparallel. The thecee are 



alternate in their arrangement, but not conspicuously 



'•-j so in the distal part of the polypary, and decidedly 



* less so than in <Uiji>1u<j. tamariscus; their average 



overlap is fully one half. In the obverse aspect the 



a. Distal thecie, as seen from the reverse . . , -, .-, 



aspect, Enlargement of specimen excavation appears to be more abrupt and the 



on same slab as PL XXX, fi"\ <Jc. -. ,. ,i i n • ,i ji 



6. Distal theeffi from the obverse aspect, ventral wall consequently less flowing than m the 



Enlargement of part of PL XXX, • i p 



fig. 9c! typical form. 



Horizons and Localities. — Llandovery, Birkhill 

 Shales, Skelgill Beds (zone of Monog. SedgivicJcii). S. Scotland: Dobb's Linn. 

 Lake District : Mosedale, Long Sleddale ; Skelgill. 



Associates, etc. — Var. incertus appears to be a fairly common fossil throughout 

 the Upper Birkhill Shales and their equivalents, especially in the zone of Monog. 

 Sedgwichii, where it occurs associated with the forms usually met with in that zone. 



Collections. — The Geological Survey of Scotland, the British Museum (Natural 

 History), Sedgwick Museum, and the Authors. 



Diplograptus (Glyptograptus) serratus, sp. nov. Plate XXX, figs. 10 a — c. 



Polypary long and relatively broad, from 4 cm. to 5 cm. or more in length, 



widening quickly at first, but subsequently more gradually to a maximum 



breadth of 3*5 mm., which is then maintained. Theca) stout, fourteen to 



eight in 10 mm., of the general Glyptograptus type, overlapping one third to 



one half their length ; apertural margins slightly undulate, excavations 



conspicuous. 



Description. — The polypary is alwaj^s of considerable size ; it widens rapidly in 



its initial portion from 1 mm. to 3 mm. and then very gradually to its maximum 



breadth of 3'5 mm., which is thereafter maintained ; in some specimens, however, 



the width never exceeds 3 mm. The thecas show a decided tendency to widen 



33 



