136 BRITISH G-RAPTOLITES. 



pronounced, and the thecal apertures are always situated within conspicuous and 

 deep excavations; and, as a general rule, not only does the apertural portion of 

 each theca show a marked introversion, but it is also introtorted. 



While all the members of the family possess these general characteristics, 

 they nevertheless exhibit great variation among themselves. Some have but a 

 gentle sigmoid curvature of their thecal walls, and a slight introversion of their 

 apertures, thus approximating in form to the Leptograptidse. Others show 

 pronounced sigmoid curvature, and possess horizontal apertures of the type 

 more especially characteristic of Climacograptus, thus suggesting relationship 

 with the Diplograptidse. 



There are only two recognised British genera belonging to this family of the 

 Dicranograptidse, namely Dicellograptus and Dicranogra/ptus. 



In the genus Dicellograptus the polypary is uniserial throughout, like that in the 

 Leptograptidse. In the genus Dicranographis, however, the polypary is uniserial 

 only in its distal portions ; proximally it is biserial, as in the Diplograptidse. 



The mode and direction of growth of the earlier thecse in the proximal portion 

 of the polypary in the species belonging to the genus Dicellograptus are similar to 

 those in Leptograptus. In some forms, however, belonging to the genus, and in all 

 forms of Dicranograptus, the mode of growth agrees with that in Diplograptus. 



Thus the Dicranograptidse may conveniently be regarded as constituting a 

 family intermediate between the Leptograptidas on the one hand, and the Diplo- 

 graptidse on the other. 



Genus DICELLOGRAPTUS, HopJcinson. 

 1871. Dicellograptm, Hopkinson, Geol. Mag., vol. viii, p. 20. 



Polypary bilaterally symmetrical, consisting of two uniserial stipes diverging 



from the sicula at angles exceeding 180°. 

 Thecae of the characteristic Dicranograptid type (ante, p. 135). 

 The polypary in Dicellograptus is generally narrow, but never so slender as that 

 in Leptograptus. The two stipes may be straight, gently flexed, or with such a 

 strong convex curvature as to cross each other ; but the polypary is always more 

 or less stiff and rigid in its general aspect, and rarely takes on the limp and 

 flexuous appearance of that of Leptograptus. 



The normal angle of divergence (ventral angle) varies greatly in the different 

 species, ranging from 220° (D. Forchammeri) , to 340° (D. anceps). The dorsal 

 angle, which is naturally the complement of the ventral angle, becomes of especial 

 diagnostic importance in this genus. It is here referred to under Hopkinson' s 



