DIMORPHOGRAPTUS. 349 



D. decussatus. 



var. parliliter. 

 D. physophora. 

 D. longissimus. 

 Group II. — Dimorpkograpti in which the biserial thecas are of the general 

 Glijptogr aphis type. 



Type, Dimorphog. erectus. 

 D. erectus. 

 D. elongatus. 



Group III. — Dimorpkograpti in which the biserial thecas are of the general 

 Amplexograptus type. 



Type, Dimorphog. extenuatus. 

 D. extenuatus. 



Group I. 

 Dimorphograpti. in which the biserial thecas are of the general Orthograptus type. 



Dimorphograptus confertus (Nicholson). Plate XXXV, figs. 3 a — d. 



1868. Diplograpsus confertus, Nicholson, Quart. Jouru. Geol. Soc, vol. xxiv, p. 526, pi. xix, figs. 



14, 15. 

 1888. Dimorphograptus confertus, Marr and Nicholson, Quart. Journ. Geol. S c, vol. xliv, p. 707, 



etc. 

 1897. Dimorphograptus Swanstoni,YSLi: Kurcki, Tornquist, On Diplograptidse and Heteropriouidse of 



Scanian Kastrites Beds, p. 19, pi. ii, figs. 33, 34. 



Polypary straight or stiffly curved, 2 — 3 cm. in length ; uniserial portion short, 



bearing three thecas ; biserial portion always straight, 2 — 3 mm. in breadth. 



Sicula conspicuous. Thecas fourteen to twelve in 10 mm., with slightly 



concave outer walls overlapping one half to two thirds their length ; 



apertural margins everted, undulate. 



Description. — The uniserial portion of the polypary has a length of about 



4 mm. ; it undergoes very marked increase in width from its origin towards 



the biserial portion, measuring "5 mm. at the level of th. I 1 and increasing to 



# 8 mm. It is, however, relatively slender compared with the biserial portion, 



which Avidens from T8 — 3 mm.; distally the margins are subparallel. 



The sicula has a length of 2 mm., reaching to the level of the aperture of 

 th. I 1 ; the virgella is sometimes conspicuous and may have a length of 1 mm. 

 or more. Th. I 1 arises slightly above the aperture of the sicula and curves 



