356 



BRITISH GRAPTOLITES. 



Figs. 233 a and 6. — Dimorphograptus 



erectus, sp. nov. 



•■ C 



thecse. Sicula long and conspicuous, with long virgella. Theca3 ten to 



eleven in 10 mm., with conspicuous sigmoid curvature, overlapping one 



third to one half their length, excavations in biserial portion wide and 



deep. 



Description. — The uniserial portion of the polypary has a length of 3 — 4 mm. 



and measures only "5 mm. in breadth; but the polypary widens to 1 mm. where the 



biserial part commences, and thence to a maximum breadth of 1*5 mm., which is 



thereafter maintained. 



The sicula is very conspicuous ; is about 2 mm. in length, and reaches to the 

 level of the aperture of th. 2 1 ; it is furnished with a long virgella which may have 



a length of 4 mm., and in one specimen shows a 

 dilatation at its extremity. Th. I 1 originates slightly 

 above the aperture of the sicula, and grows upward 

 and outward closely adpressed to the side of the 

 sicula ; th. 2 1 and th. 3 1 develop from the initial part 

 of the theca next below ; from th. 3 l (or more rarely 

 th. 4 1 ) two thecse arise, th. I 3 from the initial portion 

 of th. 3 1 or th. 4 l , and th. 4 1 or th. 5 1 from the 

 apertural region. 



The theca? measure ten to eleven in 10 mm. and 

 have an average length of 2 mm., but while in the 

 uniserial part they overlap one third of their length, 

 in the biserial portion the overlap increases to one 

 half. The thecge of the biserial portion closely 

 resemble those of Glyptog. tamariscus. 



Affinities. — A characteristic feature of the poly- 

 pary in both the species included in this group is its 

 gradual increase in breadth, there being no sudden 

 increase where the biserial portion commences, but 

 a steady widening throughout from the initial portion of the uniserial part until 

 the maximum breadth is attained in the biserial portion. From Dimorphog. 

 extenuatus it is readily distinguished by the shortness of the uniserial portion. 



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

 Stockdale Shales (zone of Dimorphog. confertus). 



Lake District : Spengill Head. S. Scotland : Dobb's Linn. Ireland : Little 

 River, Pomeroy. 



Associates, etc. — Dimorphog. erectus is a fairly common fossil in the zone of 

 Orthog. vesiculosus of S. Scotland, where it is found associated with Dimorphog . 

 confertus and var. Sivanstoni, Orthog. vesiculosus, and Glimacog. rectangularis. In 

 the Lake District it has been found in the D. confertus band at Spengill Head. 

 Collections. — Sedgwick Museum, Marr, and Elles. 



a. Proximal end, obverse aspect, show- 



ing sicula and long virgella. Dobb's 

 Linn ; Birkhill Shales (zone of 

 Or/hog. vesiculosus). Coll. Elles. 



b. Proximal end, reverse aspect, show- 



ing virgella dilating into vesicle at 

 its extremity. Ibid. 



