THE AEENIG AND LLANDEILO EOCKS OF ST. DAVID'S. 645 



and outer margins generally compressed into a long acute 

 denticle. 



Numerous imperfect examples of this species were collected some 

 years since by Professor Nicholson from the Skiddaw Slates of the 

 Lake district, and by him provisionally referred to D. serratulus, 

 Hall (Ann. & Mag. Nat. Hist. loc. cit. supra). Our better-pre- 

 served specimens, from Llanvirn, fully bear out his suspicions of 

 its distinctness, and we have now sufficient evidence to describe 

 it as new. 



It differs from D. serratulus in the number and shape of its hy- 

 drothecse. In that species there are but sixteen in the space of an 

 inch, and the apertural margin of each is straight and almost at 

 right angles with the line of the branch. In D. Nicholsoni the thecse 

 average twenty-six to the inch, and the deeply concave apertural 

 margin is prolonged normally into a long oblique denticle. 



It is named after its eminent discoverer and describer, Prof. H. 

 A. Nicholson, of the Newcastle College of Science. 



Log. Upper Arenig, Llanvirn Quarry. 



Didymogeaptus aefinis, Nicholson. PI. XXXIII. figs. 6a-6c. 

 1863. Didymograpsus, sp., Salter, Quart. Journ. Geol. Soc. vol. xix. 

 p. 137, fig. 13d. 



1869. Didymograpsus affinis, Nich. Ann. & Mag. Nat. Hist. ser. 4, 



vol. iv. p. 240, pi. xi. fig. 20. 



1870. Didymograpsus affinis, Nich. ib. vol. v. p. 343, fig. 4. 



Branches very slender, from half to three quarters of an inch in 

 length, diverging from each other at an angle of from 210° to 

 270° ; sicula slender, tapering to a point ; hydrothecaB about 18 

 to the inch, inclined to the axis at an angle of from 15° to 20°, 

 three or four times as long as wide, and with their outer 

 margins free (apparently) for the whole of their length ; aper- 

 tural angle about 120°, margin slightly convex; denticles 

 slightly mucronate. 

 Only one or two specimens showing the two branches have been 

 seen, the species usually occurring as single branches looking ex- 

 actly like fragments of Monograptus jtfilssoni, Barr., as in the Skid- 

 daw Slates, in which alone the species has before been found. 



The only perceptible difference between our specimens and those 

 from the Skiddaw series is in the form and angle of the apertures of 

 the thecaB, which in the Skiddaw specimens are at right angles with 

 the general axis of the branches, and have not the convex margin of 

 the St.-David's forms. The convexity, however, is very slight, and 

 is changed to a concavity, only just perceptible, at the distal sub- 

 mucronate extremity of the thecse. 



Loc. Upper Arenig, Porth Hayog, Ramsey Island. 



Didtmogeaptus EijoDTJS, Lapw., sp. nov. PI. XXXV. figs, la-1 c. 

 Branches rigid, of great length, proceeding in opposite directions in 



