222 BRITISH GRAPTOLITES. 



in section, and the apertural margins P. pal mens. 



even ami oblique. var. tenuis. 



var. lata. 



var. ovato-clonijattis. 

 P. minor. 

 ? P. phylloides. 

 P. altissimus. 

 Group 11. — Petalograpti in which {= Gephalograptus.) 

 the thecas were approximately straight, Type Gephalog. cometa. 

 circular in section, and with even G. cometa. 



apertural margins. G. petalum. 



C. acuminalus. 



It will be obvious from a study of the characters of the specific forms included 

 in these sub-genera in the following pages that the divisions recognised are separated 

 from each other by no hard and fast lines. The typical species in each " sub- 

 genus " are no doubt distinct as such, but there are so many divergences from 

 them in different directions within the sub-genus that there seems to be an 

 almost insensible passage from one into the other. Not only so, but some of the 

 more typical forms of the various sub-genera recall forms belonging to other 

 groups Avhich have long since generally been accorded a distinct generic rank. 



All the species included in the sub-genus Petalograptus strikingly suggest in 

 the outlines of their polypary and thecal the much older genus Phyllograptus, 

 but shade gradually in their outward aspect into those species of Orthograptus 

 grouped around 0. truneatus. Most of the forms united under Glyptograptm agree 

 with those of the genus Climacograptus in the approximately cylindrical form of the 

 polypary, but some (G. sinnatns, etc.) recall the Dicranograptidas in the characters 

 of their thecaa, while others, again (G. dentatus, etc.), seem to grade into those 

 OrthograpU typified by G. calcaratus. 



The Mesograpti pass on the one hand through M.foliaceus into the Orthograpti 

 typified by 0. calcaratus, while on the other hand such extreme forms as M. 

 perexcavatus appear to lead into the Glimacograpti. In the Orthograpti, in addition 

 to the gradations noted above we find forms grouped around 0. quadrimucronatus, 

 shading almost imperceptibly into the family of the Glossograptida\ 



Sub-genus Orthograptus, Lapworth. 



1873. Orthograptus, Lapworth, Classification of the Rhabdophora, G-eol. Mag., vol. x, p. 500. 



Within the sub-genus Oi'thograptus there is a certain amount of variation, not 

 only in the form of the polypary itself, but also in that of the thecae. Broadly 



