DIMORPHOGRAPTUS. 



357 



Figs. 234 a and 6. — Dimorphograptus 

 elongatus, Lapw. 



Dimorphograptus elongatus, Lap worth. Plate XXXV, figs. 11 a — c. 



1876. Dimorphograptus elongatus, Lapworth, Geol. Mag., p. 547, pi. xx, figs. 12 a, b. 



1877. Dimorphograptus elongatus, Lapworth, Proc. Belfast Nat. Field Club, p. 132, pi. vi, fig. 6. 



Polypary 3*5 — 5 cm. in length; uniserial portion very long and curved; biserial 

 portion approximately straight, with a maximum breadth of 1/5 mm. 

 Sicula small, virgella long and conspicuous. Thecae eight in 10 mm., of 

 the general Glyptograptus type; apertural margins introverted with acute 

 denticle, situated in shallow excavations occupying one quarter of the 

 breadth of the polypary. 

 Description. — The uniserial portion of the polypary is much longer than in any 

 other Dimorphograptus, being fully 2 5 cm. in length, and it widens gradually from 



- 3 mm. near the sicula till the breadth of 1'5 mm. 

 is attained. 



The sicula has a length of nearly 2 mm., and 

 th. I 1 appears to originate from a point half way 

 along its length. 



There are commonly seventeen theca3 in the 

 uniserial portion, th. 17 1 giving origin to the first 

 two thecae of the biserial portion ; in this the thecae 

 are alternate in their arrangement, and their aper- 

 tural margins seem to become more and more 

 introverted, and some near the distal end of the 

 polypary appear to be also introtorted. 



Affinities. — When fully grown D. elongatus is a 

 well-characterised and easily recognisable species. 

 Superficially it somewhat resembles D. extenuatus, 

 but differs in the curvature and length of the 

 uniserial portion and in the characters of the thecae. 

 Fragments of it are very hard to identify, for the 

 thecae of the uniserial portion closely resemble those of Monog. tenuis, and those 

 of the biserial part are not unlike those of Gephalog. (?) acuminatus, which occurs 

 in association with the present species. 



Horizon and Localities. — Lower Birkhill Shales (zones of Gephalog. [?] acumi- 

 natus and Orthog. vesiculosus) ; Stockdale Shales (zone of Dimorphog. confertus). 



Lake District : Skelgill. 8. Scotland : Dobb's Linn ; Craigmichan. Ireland : 

 Coalpit Bay, Donaghadee (?). 



Associates, etc. — D. elongatus is a rare fossil at the base of the Birkhill Shales 

 and Stockdale Shales. It occurs associated with G&phalog. (?) acuminatus, 

 Dimorphog, confertus and var. Swanstoni, Orthog. vesiculosus, and Monog. tenuis. 

 Collections. — Lapworth and the Authors. 



a. Proximal extremity, reverse aspect, 



showing' sicula and long virgella. 

 Enlargement of part of PI. XXXV, 

 fig. 116. 



b. Distal thecae. Enlargement of part 



of PI. XXXV, fig. 11 a. 



