358 BRITISH GRAPTOLITES. 



Group III. 

 Dimorphograpti in which the thecse are of the general Amplexograptns type. 



Dimorphograptus extenuatus, sp. nov. Plate XXXV, figs. 10 a — e. 



Polypary 2 — 3 cm. or more in length ; imiserial and biserial portions both 

 sinuons; maximum breadth 1*6 mm. Sicula long, with conspicuous virgella. 

 Thecse ten to thirteen in 10 mm. of the general Amplexograptns type, with 

 sharp sigmoid curvature ; apertural margins opening within semi-elliptical 

 excavations, occupying nearly one half the breadth of the polypary. 

 Description. — The imiserial portion has a length of 8 mm. ; it is slender, 

 measuring only - 3 mm. in breadth at its origin, and widening to *4 mm. close to the 



biserial portion, so that the general increase in 



Fig. te5.-Bimor^gra P tus extenuatus, ^^ ^ yery graduaL The biseria l portion 



, measures '7 mm. opposite the aperture of th. I 2 , and 



widens steadily until the maximum breadth is 

 attained. 



The sicula has a length of 2'5 mm. and reaches 

 nearly to the level of the aperture of th. 2 1 ; the 

 virgella is often 2 mm. in extent. Th. I 1 originates 

 at a point some little distance above the aperture of 

 the sicula, which is thus free in the whole of its 

 apertural region . There are commonly five thecce 

 in the imiserial portion. 



Proximal extremity, reverse aspect. In tlie biserial portion the thecae are shorter and 



Enlargement of part of pi. xxxv, measure thirteen in 10 mm., as opposed to ten in 



10 mm. in the imiserial part, and they overlap for 

 about one third of their length, which does not exceed 1'4 mm. The distal parts of 

 their ventral walls are free and almost vertical, so that they resemble those of 

 Amplexog. perexcavatus, and come very close in form to those of a typical Climaco- 

 graptus. 



Affinities.- — The characters of its thecse are sufficient to distinguish I). extenuatus 

 from all other Dimorphograpti. 



Horizon and Localities. — Lower Birkhill Shales (zone of Orthog. vesiculosus). 



8. Scotland: Dobb's Linn. Ireland: Coalpit Bay, Donaghadee. Side Valley, 

 Little River, Pomeroy. 



Associates, etc. — D. extenuatus occurs as a rare fossil in the Orthog. vesiculosus 

 zone of the Lower Birkhill Shales in S. Scotland and N. E. Ireland. It is 

 commonly associated with Orthog. vesiculosus and Climacog. rectangularis. 



Collections. — Museum of Belfast Natural History Society, and Elles. 



