LASIOGRAPTUS. 



323 



Var. bimucronatus (Nicliolson). Plate XXXIII, figs. 8 a — e. 



1869. Diplogrcqitiis bimucronatus, Nicholson. Ann. Mag. Nat. Hist. [4], p. 236, pi. xi, figs. 



12 and 12 1 . 

 1877. Dijolograjotm (Hallogrcqrius) bimucronatus, Lapworth, Proc. Belfast Nat. Field Club, p. 134, 



pi. vi, fig. 23. 

 1908. Lasiograptus bimucronatus, Ruedemann, G-rapt. New York, pt. 2, p. 481, pi. xxix, figs. 



12 — 18; pi. xxx, figs. 6 — 8; pi. xxxi, fig. 4. 



Fig. 212 a. — Hallograptus mucronatus, 

 var. bimucronatus (Nicb.). 





Specimen showing- scopulaB and also the 

 thecal apertures. Wanlock Head, 

 Glenkiln Shales. Coll. Lapworth. 



Fig. 212 6. — Hallograptus mucronatus, 

 var. bimucronatus (Nich.). 



This well-known form was originally distinguished and described by Nicholson 

 as a distinct species, under the title Diplograptus bimucronatus, but it presents so 



many features in common with the type form of 

 Hallog. mucronatus that we consider it better to 

 regard it as a variety only. It differs from H. mucro- 

 natus mainly in having its thecas much more closely 

 set (sixteen to twelve in 10 mm.), especially in the 

 proximal part of the polypary. In this variety the 

 paired nature of the spinous processes is often well 

 shown, and they sometimes appear as if united by a 

 membrane in the earlier part of their length. 



The polypary varies very much in size ; it some- 

 times attains a length of 5 cm. or more, but specimens 

 with a length of about 3 cm. are of most frequent 

 occurrence. It widens more rapidly than in the 

 typical form, and there is a tendency in some ex- 

 amples for the polypary to narrow again somewhat 

 in the direction of the distal end. 



Occasional specimens, which almost certainly 

 belong to this form but are preserved in scalariform 

 view, show distinct scopulate processes arising from 

 the septal strands and extending outwards for some 

 distance beyond the thecal margin of the polypary. 

 They are apparently arranged in pairs, and occur 

 at distances answering to the position of alternate 

 thecge. As a general rule, each process seems to 

 consist of two main fibrous stems about three times 

 the length of the width of the polypary. These 

 stems are irregularly curved and throw off at 

 intervals minor branches or branchlets which sup- 

 port between them a membranous film, the outer edges of which die away 

 insensibly into the surrounding rock. 







Fragment showing lateral and extra- 

 distal scopuloe. Wanlock Head, 

 Glenkiln Shales. Coll. Lapworth. 



