648 J. HOPKISTSON AND c. lapwokth on the geaptolites oe 



degrees, three times as long as wide, overlapping each other 

 for about half their length; apertural margins straight, 

 forming an angle of 90 degrees or less with the axis ; denticles 

 pointed, submucronate. 



In addition to the typical form of this species as figured by Hall 

 (Grapt. Quebec Group, pi. i. fig. 20), there frequently occurs a 

 dwarf variety with very short branches and closely set denticles. 

 It passes beyond the limits of the Upper Arenig as high as the 

 Middle Llandeilo of Abereiddy Bay. It occurs abundantly in the 

 highest beds of the Upper Arenig (Upper Skiddaw Slates) of the 

 Lake district, and may appropriately be called var. nanus. 



Loc. Typical form — Upper Arenig, Porth Hayog, Eamsey Island. 



Yar. nanus — Upper Arenig, Llanvirn Quarry ; Lower and Middle 

 Llandeilo, Abereiddy Bay. 



Dieymogeaptes Murchisoni, Beck, sp. PI. XXXV. figs. 2 a-2f. 



1839. Graptolithus Murchisoni, Beck, Murch. Silur. Syst. pi. xxvi, 



fig. 4. 

 1861. Didymograpsus Murchisoni, Baily, Quart. Journ. Geol. Soc. 



Dublin, vol. ix. pi. iv. figs, la, b, c. 



1869. Didymograpsus Murchisoni, Hopk. Journ. Quekett Micr. Club, 



vol. i. pi. viii. figs. 6a, 6b. 



1870. Didymograpsus Murchisoni, Nicholson, Ann. & Mag. Nat. 



Hist. ser. 4, vol. v. p. 349, pi. vii. figs. 7, 7 a, 7 b. 



Branches robust, from two to three inches in length, diverging from 

 a large blunt sicula at a primary angle of about 320 degrees ; 

 curved at their origin, but almost immediately becoming straight 

 and perfectly parallel ; very narrow at their commencement and 

 gradually expanding throughout to a maximum diameter of 

 one sixth of an inch; hydrothecse 20 to 25 to the inch, 

 making an average angle of 45 degrees with the axis of the 

 branch ; apertural and outer margins concave, forming a very 

 acute denticle. 



The axillary portion of the polypary in adult and typical 

 examples of this species is much broader than in any of the closely 

 allied forms ; and the sicula is proportionately stouter and shorter, 

 rarely exceeding one tenth of an inch in length, and with its distal 

 extremity either abrupt or obtusely rounded. The latter, however, 

 is not an invariable characteristic, as pointed examples are some- 

 times met with. The general angle formed by the ventral margins of 

 the branches near their common point of origin varies from 30 to 60 

 degrees. After freeing themselves from the broad curve of their proxi- 

 mal extremities, the branches become almost immediately straight 

 and parallel, and so continue for the remainder of their length. In 

 a few full-grown examples the distal portions of the branches gra- 

 dually approximate until their ventral margins are in contact, or 

 even slightly cross each other. Near the axil the hydrothecae 

 scarcely overlap each other at all, but lie almost parallel with the 



