348 BRITISH GRAPTOLITES. 



Thus, from tlio systematic point of view, the Dimorphograptidaei may be 

 regarded as constituting a family intermediate between the Diplograptidae and the 

 Monogfaptidae. 



The individuality of the family was first recognised by Tullberg, for whose 

 original title of Heteroprionidae we have here substituted that of Dimorphograptidse 

 in order to bring it into harmony with the names of the other families of the 

 Graptoloidea. 



Genus DIMORPHOGRAPTUS, Lapworth. 



1876. Dimorphograptus, Lapworth, Geol. Mag., dec. 2, vol. iii, p. 545. 



Polypary uniserial proximally, becoming biserial distally ; test membranous, 

 continuous. 



ThecdB of the biserial portion of the general Diplograpbus type; those of the 

 uniserial portion of the Monograptus type. 



The polypary is somewhat small as a general rule, only two species so far as 

 known exceeding 3 cm. in extent. It may be curved or straight, and is biserial 

 for fully one half of its entire length. The proximal uniserial portion may be 

 short or long, bearing from one to seventeen thecas, and when it is curved its 

 theca3 appear to be always on the convex side of the curve ; fragments of this portion 

 are indistinguishable from some Monograpbi. 



There appears to be a certain amount of torsion in the polypary in some species, 

 so that the uniserial and biserial portions do not grow quite in the same plane. 



The sicula is conspicuous from its position ; it is free for its entire extent on 

 one side, but is more or less embraced by th. I 1 on the other, according as the 

 first theca arises near the aperture of the sicula or near its apex. As a rule the 

 virgella is well shown. 



The course of the virgula is easily followed ; it lies on the outer edge of the 

 polypary in the uniserial portion and passes into the middle of the biserial portion. 



The thecas in the biserial portion of the polypary are of the general Diplo- 

 graptus type, but exhibit minor differences amongst themselves of much the same 

 nature as those which have been noticed within that genus; and consequently the 

 Dimorphograjpti as a whole are capable of being similarly divided into minor 

 groups characterised by the differences of the thecse. These minor groups are as 

 follows : 



Group I. — Dimorphograpti in which the biserial thecas are of the general 

 Orthograptus type. 



Type, Dimorpliog. confertus. 

 1). confertus. 



var. Swanstoni. 



