DIMORPHOGRAPTUS. 353 



Associates, ete.—D. decussatus occurs somewhat rarely in the highest beds of the 

 zone of Orthog. vesiculosus associated with Gliwacog. rectangularis and Monog. tenuis. 

 Collection.— Elles. 



Var. partiliter, var. nov. Plate XXXV, fig. 6. 



A variety which differs in some important respects from the typical form 



Fig. 230.-Dimorphogra P tus decussatus, occurs on approximately the same horizon. In 

 var. partiliter, nov. this varietj fcne un i ser ial portion is longer and 



j^f?, its thecee are not so separated from each other, only 



the apertural portions of each being completely 

 isolated. The biserial thecse are also more distant, 

 numbering only ten in 10 mm. As in the polypary 

 of the typical form, however, the maximum breadth 

 of 1*5 mm. is attained almost at the commencement 

 of the biserial portion. 



Horizon. — Lower Birkhill Shales (zone of Orthog. 

 t, , „ , „ vesiculosus). 



Keverse aspect. Enlargement of part ' 



of n. xxxv, fig. 6. Locality.— Dobb's Linn. 



Collection. — Elles. 



Dimorphograptus physophora (Nicholson). Plate XXXV, figs. 7 a—d. 



1868. Diplograptus physophora, Nicholson, Ann. and Mag. Nat. Hist. [4], vol. i, p 56, pi. iii, fig. 7. 

 1880. Diplograptus physophora, Lapworth, Ann. and Mag. Nat. Hist. [5], vol. v, pi. v, fig. 26. 



Polypary 1 — 3 cm. in length and with maximum breadth of 2 mm., uniserial 



portion very short and. straight, biserial portion also straight, proximal 



end enclosed in a disc ; sicula long, conspicuous. Thecas nine to seven 



in 10 mm., overlapping one half to two thirds their length; apertural 



margins approximately horizontal. 



Description.- — The uniserial portion of the polypary in this species is practically 



reduced to a minimum, for it is composed of only two thecge. Opposite the 



aperture of th. I 1 the polypary measures *7 mm. in breadth ; it increases up to 



1*5 mm. at the commencement of the biserial portion, and thence widens gradually 



till a breadth of 2 mm. is attained, when the margins continue subparallel for the 



remainder of their extent. 



The sicula has a length of 1'5 mm. and reaches well up into the biserial portion 

 of the polypary, terminating approximately on the level of the aperture of th. 2 1 ; 

 th. I 1 originates slightly above the aperture of the sicula and has the appearance 

 of growing at first slightly downward, but it quickly bends round, grows upward 



46 



