510 Miss G. 1. ELtES ON TfiE GEAl?TOLlTE [Aug. 1 898, 



(n) DiDYMO&EAPTTJS Y-EEACTTJS Vai. VOLUCEK, H. 0. Nich. 



1890. Didymograptus V-fractus var. volucer, H. 0. Nich. Geol. Mag. dec. iii, 

 vol. vii, p. 342, fig. 3. 



This variety agrees in general characters with the foregoing 

 species, but the whole rhabdosoma is thicker, the basal angle 

 narrower, and the Y-part more prolonged. The bend is very 

 abrupt, and the stipes thereafter run in a direction at right angles 

 to their previous direction of growth. 



ISotes on Structure. — The sicula appears to resemble that of 

 Didymograptus V-fractus both in position and form, and the details 

 of the proximal end, so far as can be made out from the only two 

 specimens known to me, seem to agree with that species. 



The stipes are narrowest at their origin ; they grow nearly parallel 

 at first, and steadily increase in width up to the bend, where they 

 are -^^ inch (2*1 mm.) wide. In the specimen in the Woodwardian 

 Museum the width appears to be greatest at the bend, and then to 

 diminish again towards the distal extremities of the stipes, but in 

 Nicholson's specimen the width appears to be maintained along the 

 horizontal part of the stipe. 



In the Y-part of the rhabdosoma the concave curvature of the 

 thecal walls is apparent, and is continued for a longer distance than 

 in D. V-fractus, being continued for the first seven thecse at least. 



The thecse number thirty-six to the inch (fourteen in 10 mm.) ; 

 in the Y-part they are in contact for half their length, but in the 

 horizontal part for fully two-thirds of their length. 



Angle of inclination : (a) Y-part = 25° to 30° ; (6) horizontal 

 part = 60°atcurveto40°. 



Apertural angle : {a) Y-part nearly perpendicular ; (6) hori- 

 zontal part= 120° to 125°. 



The apertures are slightly mucronate. The thecae are about 3 

 times as long as wide. 



Occurrence (?). Locality. — Outerside. 



{g) DiDTMOGKAPTUS IITOENTUS (Hall). 



1858. Grraptolithus indentus, Hall, Greol. Surv. Canada Eep. 1857, p. 128. 



1865. G-raptolithus indentus, Hall, ' Grapt. of the Quebec Group,' Geol. Surv. 

 Canada, dec. 2, p. 74 & pi. i, fig. 20. 



1875. Didymograjptus indentus, Lapw. & Hopk. Quart. Journ. Geol. Soc. vol. xxxi, 

 p. 647 & pi. xxxiii, figs. 7 a-c. 



All the specimens of D. indentus (Hall) and the variety nanus 

 from the Skiddaw Slates are badly preserved as regards the thecae, 

 and it is therefore in some cases rather difiicult to distinguish 

 between them, though the forms that are less than | inch (12-7 

 mm.) long should probably be referred to the variety. When the 

 number of thecae can be counted the distinction can more readily be 

 made. Both forms would seem to be present in the Skiddaw Slates, 

 and to occur together in great abundance at certain localities, as, 

 for instance, at Outerside and the Grlenderamakin River. 



In both forms the sicula is long and narrow, and occasionally 

 shows the thread-like virgula proceeding from its apical extremity. 

 The initial angle of divergence of the stipes is generally about 60°, 



