35 i 



BRITISH GRAPTOLITES. 



and outward. Tli. 2' arises from its apertural region, and from this two thecge 

 are developed and the biserial portion is originated. In the reverse aspect 



of the polypary it can be clearly seen that th. I 2 

 grows from the initial portion of th. 2 1 , while th. 3 1 

 grows from its apertural region. Owing to this 

 growth of th. 1- a considerable portion of the 

 sicula is concealed in this aspect. 



The whole initial region of the polypary, in- 

 clusive of the base of the biserial portion, is fre- 

 quently, though not invariably, enclosed in a disc- 

 like body which is approximately circular in form, 

 having a diameter of about 2*5 mm. 



The thecas number ten in 10 mm. ; they have 

 an average length of 3 mm., and the overlap 

 increases from half to two thirds in the more distal 

 parts of the polypary ; the free outer walls are 

 nearly straight, and the apertural margins are in 

 general slightly concave ; at first they show a 

 tendency to slight e version, but quickly become horizontal. 



Affinities. — -D. jjlij/sojihora is the only Diniorphograptus in which a disc has 

 hitherto been found, and when that is preserved it is easily separable from all 

 other forms. When it is not present the polypary exhibits some features which 

 it shares with I), erectus. It is, however, distinguishable by its extremely short 

 uniserial portion and the general rigidity of the polypary. 



Horizon and Locality. — Lower Birkhill Shales (zone of Orthog. vesiculosa*). 

 S. Scotland : Dobb's Linn. 



Associates, etc. — D. physophora occurs in some abundance at the top of the 

 Orthog. vesiculosus zone (sub-zone Monog. tenuis), associated with 0. vesicidosus, 

 Olimacog. rectangularis, 01. innotatus, and Monog. tennis. 

 Collections. — Lapworth and the Authors. 



Figs. 231a and b. — Dimorphograptus 

 physophora (Nich.). 





v 

 1 



a. Obverse aspect, showing sicula and 



disc. Enlargement of part of speci- 

 men on same slab as PI. XXXV, 

 fig. 7 d. 



b. Distal thecae. Enlargement of part 



of specimen on same slab as PI. 

 XXXV, figs. 7 a and 7 c. 



Dimorphograptus cj'r. longissimus (Kurck). Plate XXXV, figs. 8 a — d. 



1881. Diplograptus (?) longissimus, Kurck, Nya G-rapt. fr. Skaue, Geol. Foren. Forhandl., vol. vi, 



p. 302, pi. xiv, figs. 8 and 9. 

 1897. Non Diplograptus longissimus, Tornquist, Diplograptidse and Heteroprionidse of Scanian 



Rastrites Beds, p. 17, pi. ii, figs. 26 — 29. 



Polypary 2—4 cm. or more in length; biserial portion long and straight, 

 having a maximum width of 2 — 2'5 mm. which is attained quickly, so that 

 margins are subparallel ; uniserial portion relatively short, 2'5 mm. in 

 length and comprising three theeas, slightly curved. Sicula conspicuous. 



