DIDYlMOaRAPTUS. 47 



Specimens are in the collections of the Woodwardian Museum, Nicholson, and the 

 Authors. 



Didymograptus nanus, Lapworth. Plate IV, figs. 5 a — ]i. 



1868. Didynioyrapsus geminus, Nicholson, Quart. Jourii. Geol. Soc, vol. xxiv, p. 134, pi. v, figs. 8, 9. 

 1870. Diilymograpsus geminus, Nioholsou (pars), Ann. Mag. Nat. Hist. [4], vol. v, p. 346, fig. 6b. 

 1875. Didymograptus indentns, var. tianus, Lapworth, Quart. Journ. Geol. Soc, vol. xxxi, p. 647, 



pi. xxxiii, fig. 7 d ; non pi. xxxv, figs. 4 a — f. 

 1898. Didymograptus indentus, var. namis, Elles, Quart. Journ. Geol. Soc, vol. liv, p. .511. 



Stipes ] 2 — 25 mm. in length, slender, maintaining an almost equal width of 

 I'o mm. throughout their length, diverging from each other at a primary 

 angle of about 90°, but gradually incurving, and running parallel for the 

 remainder of their extent. Sicula conspicuous but slender. Tliecse ten to 

 twelve in 10 mm., inclined at 30°, three and a half times as long as wide, 

 free one half to one third their length. Apertural margins normal, 

 slightly concave, denticles submucronate. 

 Description. — The dorsal walls of the stipes appear in general to be 4 mm. 

 apart, and the stipes are commonly 1 mm. in width. 



The sicula is about 2*4 mm. in length ; it is slender, and often shows a filiform 

 nema proceeding from its apical extremity. I'h. 1^ originates slightly above the 

 aperture of the sicula, and grows away, leaving the sicula free on the left side 

 (obverse view) for a short distance. The crossing canal is slender and ol)lique. 

 A marked want of symmetry is observable as regards the origin of the stipes. 



The number of thecse in a given unit of length 

 Figs. 29 a and h.— Didymograptus varies in different specimens ; in those from 



nanus, Lapwortli. i • i 



S. Wales the number m the mature part of the 



stipe is commonly about ten in 10 mm., but about 



eleven in 10 nnn. nearer the sicula. In tlit 



specimens from the Lake district (originally 



identified by Nicholson with J). (jemiiiHH) the theca3 



are poorly preserved, but when seen, appear to be 



^( J, set rather more closely, numbering about eleven to 



a. Proximal end, reverse view. En- twelve in 10 mm., though they agree with the 



krgement of part of PI. IV, ^y^j^j^ specimens in all other particulars. Each 



6. Prolciuiai end, obverse view. Liaii- ^lieca measures about 2 mm. in length. 



van Quarry. Coll. E. M. R. ° 



Wood. The species is distinguished by the following 



features : 



(1) the parallel growth of the stipes, 



(2) their uniform width, 



(3) the characters of the thecae. 



