220 



BRITISl I GRAPTOLITES. 



the apertural margins of the thecae transgress from one edge as narrow slits for 

 the whole of their depth upon the body of the polypary (Fig. 145c). Sometimes, 

 as in O. vesiculosus, the thickened apertural margins of the row of theca3 upon the 

 opposite edge show through the test of the compressed polypary from the buried 

 face below, and there is presented what at first sight appears to be a bi-scalari- 

 form view (PI. XXVIII, fig. 8 c). 



Classification. — The major classification of the British forms of Diplograptus 

 adopted here follows essentially on the lines of that suggested by Lapworth in 

 1873; the minor grouping corresponds with that already employed by us in the 

 case of the genus Glimacoijraptus (p. 183). 



A. — Dijplograpti in which the polypary was almost square or quadrangular in 

 transverse section. 



= Orthograptus, Lapworth. 



Group I. — Orthograpti in which the 

 thecse were practically rectangular in 

 section, and the apertural margin lobate, 

 often spinose; excavation fairly distinct. 



Group II. — Orthograpti in which 

 the thecas were almost semi-circular in 

 section, and the apertural margin plain 

 or but slightly lobate ; excavation incon- 

 spicuous. 



Group III. — Orthograpti in which 

 the thecee were sub-rectangular in sec- 

 tion, and the apertural margins strongly 

 lobate ; excavation distinct. 



Type Orthog. quadrimucronatus. 

 0. quadrimucronatus. 



var. spinigerus. 

 0. Pageanus. 



var. micr acanthus. 



var. abnormispinusus. 

 0. Whitfieldi. 

 0. insectiformis. 

 0. vesiculosus. 



var. penna. 

 0. mutabilis. 

 0. bellulus. 

 Type Orthog. truncatus. 

 0. truncatus. 



var. abbreviatus. 



var. intermedins. 



var. pauperatus. 



var. socialis. 

 0. ajperoides. 

 Type Orthog. calcaratus. 

 0. calcaratus. 



var. basilicus. 



var. vulgatus. 



var. acutus. 



Y&v.pviscus. 

 0. rugosus. 



var. apiculatus. 



