272 BRITISH GRAPTOLITBS. 



Figs. 187 e and f.-Ampiexograptus mine its exact length. The virgella is very long, 



being fully 8 mm. in extent, with a membrane in its 



initial portion ; the spines on the basal thecse are 



j 1 mm. in length in the obverse aspect of the poly- 



\ J / pary, but are somewhat foreshortened in the reverse 



aspect. The thecas are of the general Amplexo- 



graptus type, but the difference in the obverse and 



i ' reverse aspects of the polypary is not so great as in 



f Amplexog. perexcavatus, and presumably, therefore, 



_,. , .,, . , , the polvpary was not so decidedly concavo-convex 



e. Distal thecse, obverse aspect, snowing r jr j J 



deep excavations. Enlargement of jj-j section, 

 part of specimen on same slab as 



Pi. xxxi, fig. 16 d. The commonest view is that showing the reverse 



f. Distal thecoe, reverse aspect, showing _ # 



their commonest appearance. En- aspect (Fig. 187/), when the free edge is seen to be 



largement of part of another speci- 

 men on same slab as pi. xxxi, somewhat rounded, and the apertural margins some- 

 what introverted, the excavation occupying one third 

 of the width of the polypary, and one half to one third of its ventral margin. 

 In the obverse aspect the excavations are more conspicuous, the free edge of the 

 thecal wall is straight, and makes a sharp angle with the impressed edge (Fig. 

 187 e). 



Affinities. — A. arctus is undoubtedly closely related to Amplexog. cwlatus, but 

 differs in its smaller size, and in having more closely-set thecae. From Amplexog. 

 perexcavatus, to which it presents some resemblances, it differs in having (a) a 

 narrower and less concavo-convex polypary, (b) rather more remote thecas, and 

 (c) a long virgella. 



Horizon and Locality. — Llandeilo-Bala, Bicranograptus Shales. 



8. Wales : Spittal Road Railway Cutting, near Clarbeston Road ; Pan-teg. 



Associates, etc. — Amplexog. arctus has a gregarious habit, and occurs in great 

 profusion in a single bed in the Lower Dicranograptus Shales, where it is 

 associated with Dicellog. sextans, Glyptog. teretiusculus, var. euglyplius, and other 

 forms. 



Collections. — Geological Survey of England and Wales, and the Authors. 



