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LASIOGRAPTUS. 321 



Lasiograptus (Hallograptus) mucronatus (Hall). Plate XXXIII, figs. 6 a — e. 



1843. Graptolithus mucronatus, Hall, Pal. New York, vol. i, p. 268, pi. lxxiii, figs. 1 a — d. 



1877. Diplograptus {Lasiograptus) mucronatus, Lapworth, Proc. Belfast Nat. Field Club, Grrapt. Co. 



Down, pi. vi, fig. 22. 

 1908. Lasiograptus mucronatus, Ruedemann, Grapt. New York, pt. 2, p. 479, pi. xxix, figs. 9 — 11 ; 



pi. xxx, figs. 1 — 5 ; pi. xxxi, figs. 1 — 3. 



Polypary lax, having a length of 3 — 5 cm. and a breadth of 3 ram. exclusive of 

 spines. Virgella conspicuous. Thecse twelve to eight in 10 mm., having a 

 length of 2*5 mm. and overlapping about one half their length ; excavation 

 prolonged almost up to the apertural margin, which is rostrate ; ventral 

 processes apparently apertural or sub-apertural in origin, long, slender, 

 flexed, and paired. 



Description. — The generally lax form of the polypary is a characteristic 

 feature of this species. It is fairly wide, measuring 1*2 mm. at its origin and 



figs. 2io« and b.-Eaiiograptus mucro- increasing at first rapidly and then more gradually 

 wains (Hail). U p ^ a w j c [^] 1 f 3 mm#j which is thereafter con- 



stant. 

 \\i; "^ The sicula has not been observed, but its posi- 



tion is indicated by the virgella, which, though short 

 I 

 '■'. -nr, and slender, is usually conspicuous, and has a length 



of '5 mm. Th. I 1 is occasionally seen to give off, in 



addition to the apertural spine, a mesial spine at 



«. Proximal end, showing- paired thecal the angle where upward growth commences. Th. 



xxxiii, fio- ai (fcT nen ° par ' 1~ grows obliquely across the sicula and possesses 



b. Distal theeS, showing thickened , rorrpqnonrHna- qrtinps 

 angles of ventral excavation pro- lwo COI1 espOIHling spilieb. 



longed into spines. Cairn Kyan A11 tl thecas have rostrate apertural margins 



Glenkdn Shales. Coll. Sedgwick 1 » 



Museum. when compressed, and each is always provided with 



a long, slender, flexed spine, the base of which is broad, and appears to be formed 

 by a gradual narrowing of the whole denticle. Although only one spine is usually 

 visible, two are occasionally shown, and there is little or no doubt that the spines 

 were invariably paired. Owing to the slender and flexed nature of these thecal 

 spines, their direction is very variable. 



It is uncertain whether any examples of this form were provided with scopulae, 

 but judging from the number of thecas in 1 mm. it is probable that the scopulate 

 specimen figured on PI. XXXIII, fig. 6 e, is referable to this form. 



Affinities. — The characteristic features of II. mucronatus are the curiously 

 loose appearance of the stipe, and the irregular direction of growth of the 

 spines. It presents at first sight some resemblance to Orthog. qiiadrimucronatus, 

 but the spines are more slender and more flexed than in that species, the whole 

 polypary is altogether less rigid and the thecse are different in form. From 



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