DIPLOGRAPTt">. 223 



speaking, it may be said that the polypary was typically square in transverse section, 

 and the thecae in the shape of more or less flattened rectangular prisms with 

 undulate or lobate apertural margins (0. quadHmucrotwUus). The polypary, 

 however, in some species tends to lose its angularity, and to be sub-rectangular 

 rather than square in transverse section ; the theca? are also less rectangular in 

 form, at the same time that the apertural margin is more strongly lobate (0. 

 calcarattis). In others a somewhat intermediate variation is brought about by 

 the gradual flattening of the polypary, while the thee a? become more semi-circular 

 in section, and the lobate character of the apertural margin diminishes to 

 insignificance (0. truncatus). 



Geoff I. 



Orthograpti, in which the theca? were practically rectangular in section, and the 

 apertural margin lobate and often spinose ; excavation fairly distinct. 



Diplograptus ^Orthograptus) quadrimucronatus (Hall). Plate XXVIII, figs. 1 a — d. 



1865. Graptolithus quadrimucronatus. Hall, Grapt. of Quebec Group, p. 144, pi. xiii, fi-rs. 1 — 10. 



1807. Diplograpsus quadrimucronatus, Nicholson, Geol. Mag., voL v, p. 111. 



1876. Diplograptus aculeatus, Lapworth, Gat. West. Scott. Foss., pi. iv, fig. 44-. 



1877- Diplograptus quadrimucronatus, Lapworth, Grapt. Co. Down, p. 133, pi. vi, fig. 20. 



Polypary 7 cm. or more in length, widening gradually within 2 cm. from 

 1*5 mm. to a breadth of about 3 mm., which is then maintained. Sicula 

 1*5 mm. in length, free on one side for a short fraction of length; lateral 

 spines present, but rarely preserved. Theca? twelve to eight in 10 mm., 

 rectangular prisms having a length of 2 mm., and overlapping one third to 

 one half their extent ; apertural margins slightly lobate, both outer angles 

 furnished with stiff projecting spines. 

 figs. i43a and b.— Orthograptns qua.iri- Description. — The polypary. which is charac- 



mv.cronatus (Hall). J * ■* r ** 



teristically stiff and rigid, is often of considerable 

 length. The increase in breadth all takes place in 

 the proximal region of the polypary, so that for the 

 greater part of its length the margins are prac- 

 ~ tically parallel. 



In the symmetrical profile view the theca? have 



a b . 



a general inclination of about 20° ; but, like most of 



a. Young specimen showing: full length , . ... 



of sicuia. On same slab as pi. the forms m this group, this species presents itself 



XXVIII, fig. 1 d. -, i j- i- • p ■ ml 



i. Proximal end of adult specimen show- under several distinct views after compression, lhe 



ing extra spines on proximal thecae. „„„x „i j. • «: • .1 i -i ■ r 



Enlargement of p^t of pi. xx vni. most characteristic is the sub-scalariform view 



(Fig. 145 <), in which the apertural spines belonging 



