40 BRITISH GRAPTOLITES. 



D. MurchUoni agrees with D. hifidus in the number of thecae in a given unit of 

 length, but differs in general shape : for, as a general rule, in I). Murchisoni the 

 stipes are more or less parallel, and widen gradually up to their maximum, which 

 is reached after about 2 cm. ; while in D. hifidus the stipes are continuously diver- 

 gent, and widen throughout their length. 



Horizon and Localities. — Lower Llandeilo (Upper Llanvirn of Hicks). 



liadnorshire : Gelli, Llandrindod. Sliropshire : Holywell Burn, Betton Dingle, 

 Stapeley. Pembrolieshire : Abereiddy Bay. 



Associates, etc. — D. Murchisoni occurs in abundance in many localities at the 

 base of the Lower Llandeilo beds, where it forms a well-defined zone. It has not 

 yet been recognised in the Lake district. It is commonly associated with 

 var. geminus, and with species usually referred to Diplograptus foliaceus and 

 (Jryptograytus tricoritis. 



Murchison's type specimen comes from Grelli, Llandrindod. Other good 

 specimens come from S. Shropshire. In both cases the containing rocks are 

 uncleaved. Numerous specimens are to be found in the collections of the 

 Geological Survey, Lapworth, and the Authors. Abundant examples from 

 Abereiddy Bay, all more or less distorted by cleavage, are found in the 

 collections of the British Museum, the AVoodwardian Museum, and of the Authors. 



The slab of rock, on which occur the specimens from which Sowerby's figures 

 and descriptions were drawn up, is in the museum of the Geological Society. The 

 examples figured in our PI. Ill, figs. 1 a and 1 b, are selected from that slab, and of 

 these fig. ] h may be regarded as the type. 



Var. geminus (Hisinger). Plate III, figs. 2 a— 7. 



1840. Prionotus gemi7ius, Hisinger, Lethea Suecica, Supplementiim Secundum. 



1850. Graptolithus geminus, Scharenberg, " Ueber Graptoliten," p. 13, pi. i, fig. 1. 



1851. Prionotus (?) yemiuns, Boeck, " Beinaerlcriuger augaende Graptolitlierue," fig. 24. 



1875. Did ijmocjr aphis fiiirillatus, Lapworth, Quart. Journ. Geol. Soc, vol. xxxi, p. 649, pi. xxxv, 



figs. 3 b, c. 

 1881. Didi/mofjrajAus Murchisoni, var. geminus (His.), TuUberg, BiMng till K. Svensk. Veteuskaps 



Akad. Handl., vol. vi, No. 13, p. 16, pi. iii, figs. 5, 6, 10. 



This variety has already been noticed in some detail in the preceding para- 

 graphs, l)ut it may be advisable to summarise here its more prominent charac- 

 teristics. 



Stipes widening more rapidly than in D. MdrcJtisoiii, never strictly parallel, 

 but generally divergent throughout the greater part of their length. 

 Diverging from each other at a primary angle of al)Out 90°, then curving 

 round so as to include an angle of 20° — 30°. Sicula conspicuous, slender. 



