68 HYDROZOA. GRAPTOLITOIDEA 



Bryograptus. Polypary bilaterally symmetrical, uniserial, 

 consisting of two main stipes diverging at a small angle from the 

 sicula, which has its point directed distally. From the inner 

 margins of the main stipes similar secondary stipes (which may 

 bear other stipes) arise. Hydrothecse like those of Dichograptus. 

 Tremadoc. Ex. B. kjerulfi. 



Leptograptus. Polypary consisting of two simple, slender, 

 flexuous, uniserial stipes given off in opposite directions from the 

 sicula at angles greater than 180°. Hydrothecae are long tubes with 

 slight sigmoid curvature, in contact for half their length. Upper 

 Llandeilo to Lower Bala. Ex. L. Jlaccidus, Lower Bala. 



Pleurograptus. Two principal branches as in Leptograptus; 

 these bear secondary branches on both sides, often arising alter- 

 nately, and sometimes bearing smaller branches. Lower Bala. Ex. 

 P. linearis. 



Nemagraptus ( = Ccenograptus). Polypary bilaterally sym- 

 metrical, uniserial, consisting of tw r o slender, more or less flexuous 

 main stipes coming off from the middle of a well-defined sicula ; from 

 each of these stipes secondary branches may be given off in a 

 symmetrical or nearly symmetrical manner. Hydrothecse as in 

 Leptograptus. Llandeilo. Ex. N. gracilis. 



DicranograptUS. Polypary bilaterally symmetrical, biserial 

 in the proximal portion, dividing distally into two uniserial branches. 

 Hydrothecse with sigmoid curvature and inturned apertures. Upper 

 Llandeilo to Lower Bala. Ex. D. clingani, Bala. 



DicellograptUS. Like Dicranograptus, but uniserial through- 

 out, the two branches united at the sicula only, which points distally. 

 Angle of divergence greater than 180°. Upper Arenig to Upper 

 Bala. Ex. D. anceps, Upper Bala. 



DiplograptUS. (fig. 13 d.) Polypary biserial. Hydrothecse 

 subprismatic or subcylindrical, overlapping and placed obliquely. 

 Virgula prolonged beyond the distal extremity of the polypary. 

 Sicula more or less completely concealed. Lower Arenig to 

 Tarannon. Ex. D. foliaceus, Llandeilo to Lower Bala. Petalo- 

 graptus and Cephalograptus are sub-genera ; Llandovery and 

 Tarannon. 



