RETIOLITID.E. 335 



distinct impression or excavation in the middle third of its ventral wall. In 

 others, again, the actual forms of the thecee are unknown, though occasionally 

 their floors are visible (Figs. 221 ft and r). 



The apertural margins of the thecee not only vary in form in the different 

 species, but also in the matter of ornament. In some (Fig. 220 c) they are 

 practically destitute of ornament, in others they are provided with ventral 

 processes in the form of a pair of apertural spines (Figs. 226 b and c) ; in others, 

 again, the ventral processes are combined into a marginal lacinia (Figs. 222 a 

 and c). No evidences of the existence of septal processes have hitherto been 

 detected in the British forms of the Retiolitidee, nor of the possible representatives 

 of these processes — the septal elevations and pores (?) shown in the continental 

 examples of Stoma tog ra phis. 



The British species belonging to the family fall into three fairly well-marked 

 groups, which are typified respectively by the species Betiolifes Gevriitziauus, Ban*., 

 B. obesu-i, Lapw.j and R. nasm, Holm. 



In the first of these groups the thecee are quadrangular or subquadrangular in 

 section, and practically destitute of ornament ; in the second group the actual form 

 of the thecee is uncertain, but the polypary is provided with a ventral lacinia ; in 

 the third group the thecal are flattened, steeply inclined, and occasionally spinose. 

 It may possibly be shown in the future that each of these groups is worthy 

 of generic rank, but in the meantime we class them as subgenera only. 



Guour I. — Betiolifes in which the thecee are quadrangular or sub-quadrangular 

 in section and practically devoid of ornament. 



= Gladiograptus, Hopkinson and Lapworth. 

 Type Gladiograptus Geinitzianus. 



var. angustidens. 

 G. perlatus. 



var. Daironi. 

 Group II. — Betiolifes in which the form of the thecee is unknown but the 

 polypary possesses a ventral lacinia. 



= Flegmatograptus, nov. 



Type Plegmatograptits nebula. 

 P. obesus. 



var. macilentus. 

 Group III. — Betiolifes in which the thecee are steeply inclined, flattened, and 

 occasionally spinose. 



= Gotliograptus, Freeh. 



Type Gotliograptus nassa. 

 G. spinosus. 



