DIDYMOGRAPTUS. 49 



Associates, etc. — It occurs associated with D. hifidvs, D. acutidens, and D. 

 Nicholsoni. Specimens are in the collections of the Woodwardian Museum, Marr, 

 Nicholson, and the Authors. The type specimen (PI. IV, fig. 6 a) is in the 

 WoodAvardian Museum. 



Didymograptus stabilis, sp. nov. Plate IV, fig. 2. 



1875. Didymograptus indentus, Hopkinson, Quart. Jouru. Geol. Soc, vul. xxxi, p. 647, pi. xxxiii, figs. 



7 a — c. 



Stipes from 4 to 5 cm. in length, fairly wide at their origin, and attaining' 



the maximum width of 1"G mm. within the first cm. of length; 



diverging from each other at a primary angle of about 90°, then curving 



in gradually so as to include an angle of about 10°, but eventually 



becoming parallel, and continuing so for the remainder of their length. 



Sicula conspicuous. Tliecae fourteen in 10 mm., slightly curved, inclination 



40°, about four times as long as wide, free one third their length. Aper- 



tural margin normal, concave, submiicronate. 



DeserijMnii. — The stipes are about •(> mm. wide at their origin, but expand 



within the first 12 nnn. to a maximum width of 1"6 mm., which is maintained; 



there is an extent of about 4 — 4%5 mm. between the dorsal Avails of the stipes, 



])ut this appears A^ery small in proportion to the length of the polypary. 



Figs. 31 a ■.uxAh.-DUhjmacjraptus ^he sicula is slender; its length is 2-5 nnn. The 



stahihs, sp. nov. proximal end of the polypary has a thickened appear- 



^;'A ance ; th. 1^ originates at a point about midway 



i^^ between the apex and the aperture of the sicula ; the 



1^% crossing canal is thick, and but slightly oblique, so 



^^^r that the relation betAveen the stipes and the sicula 



%y^ is symmetrical. Othei- details are obscure. 



4^ Affinities. — This foi'm Avas originally figured and 



b described (Hopk. and l^apAA., ' Quart. .Jouru. Geol. 



» Proximal end, reverse view (?). SoC.,' Vol. XXxi, p. 047, 1)1. XXxiv, fio-,s. 7 d — c) aS D. 

 Enlargement of part of PI. IV, . . ^ ' ^ ' '=' ^ 



fig. 2. indentus, Avhicli has, however, a far smaller number 



b. Distal part of same. j- J_^ • .i ■, p ^ ,i ^ 



or tliecas m the same unit ot length, as shown 

 in the above description. It more closely resembles a narrow form of D. 

 Murchisoni. 



Horizon and Localities. — Upper Arenig (Llaiiviru). 

 )S7. David's District : Porth-hayog, Ramsey Island. 



Associates. — D. .stalnlis occurs associated with B. bifidus, D. acutidens, D- 

 Xicholsoni, etc. All the specimens at present known are in the collection of the 

 NVoodAvardian Museum. 



7 



