138 



BRITISH GRAPTOLITES. 



others (D. anceps) the spine is situated about midway between the aperture and 

 the " excavation." 



All the thecas in Dicellog ra/plus are of the general type characteristic of the 

 family. They present, however, many differences of detail in the several species, 

 varying in (1) the amount of curvature of the ventral walls, (2) the amount of 

 introversion and introtorsion of the free apertural portion, (3) the inclination of 

 the aperture, and (4) the amount and shape of the ventral excavation. 



In the various genera of the Dichograptidas the form of the theca remains 

 practically constant, while the form of the polypary exhibits great variation, and 

 the recognised minor groups in that family are consequently founded upon the 

 latter characteristics. In the genera of the Dicranograptidas, on the contrary, 

 the form of the polypary remains more or less invariable, while the thecge show 

 many distinctions in matters of detail, and we consequently utilise these for our 

 minor grouping. 



I. — Dicellograpti in which the thecas 

 have straight ventral walls and horizon- 

 tal apertures. 



Type Dicellog. complanatus. 

 D. complanatus. 



var. omatus. 

 D. anceps. 



II. — Dicellograpti in which the thecas 

 have approximately straight ventral 

 walls, but slightly introverted apertural 

 portions. 



Type Dicellog. divaricatus. 

 D. divaricatus. 



var. rigidus. 



var. salopiensis. 

 D. intortus. 



III. — Dicellograpti in which the thecas 

 narrow aperturally, have gently curved 

 ventral walls, and slightly introverted 

 apertural portions. 



Type Dicellog. patulosus. 

 D. patulosus. 

 D. pumilus. 

 D. angulatus. 



TV. — Dicellograpti in which the thecas 

 have markedly curved ventral walls and 

 strongly introverted and introtorted 

 apertural portions. 



Type Dicellog. Morrisi. 

 D. sextans. 



var. exilis. 

 D. Forchammeri. 



var. jiexuosus. 

 D. Morrisi. 

 D. moffatensis. 

 D. elegans. 



var. rigens. 

 D. caduceus. 



