656 J. HOPKINSON AND C. LAPWORTH ON THE GRAPTOLITES OF 



shape of the last in casts of this form is strikingly distinct from 

 that of almost every other described species of Glimacograptus, and 

 shows that in their uncompressed state the hydrothecae must have 

 greatly resembled, both in their disposition and in their general 

 form, those in the subgenus Glyptograptus. A farther distinction 

 consists in the occasional protrusion of the epiderm of each hydro- 

 theca immediately at its commencement, thus forming a rudimentary 

 proximal spine. 



Log. Upper Arenig, Porth Hayog, Ramsey Island. 



Genus Diplograptus, M c Coy. 



Diplograptus dentatus, Brongniart, sp. PL XXXIV. figs. 5 a-5 Tc. 



1828. Fucoides dentatus, Brongniart, Hist.Veg.Foss.pl.vi. figs. 9-12. 

 1865. Diplograptus pristiniformis, Hall, Grapt. Quebec Group, p. 110, 



pi. xiii. figs. 15, 16, 17. 

 1867. Diplograpsus pristiniformis, Nicholson, Quart. Journ. Geol. 



Soc. vol. xxiv. p. 140, pi. v. figs. 14, 15. 



Length one inch to one inch and a quarter, maximum width one 

 tenth of an inch ; ventral margins normally parallel, converging 

 near the rounded proximal extremity, which is furnished with a 

 strong pointed radicle ; virgula stout, distally prolonged ; 

 hydrothecse 22 to 30 to the inch, making a small angle with 

 the general line of the septum ; outer margin of each convex, 

 apertural margin deeply concave when compressed, and some- 

 times provided with a single median spine which is directed 

 distally. 



The compressed polypary in this species presents a great variety 

 of appearances, dependent upon age Or direction of compression. In 

 the normal (reverse) aspect the deeply concave apertural margins 

 and the broadly rounded outer edges of the hydrothecae are very 

 characteristic. Examples^-presenting the opposite (obverse) view 

 cannot be distinguished from young specimens of D. foliaceus, 

 Murch.; while those offering the ventral or scalariform aspect 

 are inseparable from Glimacograptus. Almost invariably the well- 

 marked radicle is the only visible proximal appendage ; but there 

 is evidence that it was sometimes accompanied by two very minute 

 lateral spines. The distinct apertural spines can be seen only in 

 the normal aspect of the polypary, and are visible only in very rare 

 cases. 



Log. Upper Arenig, Porth Hayog, Eamsey Island, and Llanvirn 

 Quarry. 



Diplograptus eoliaceus, Murchison, sp. PI. XXXV. figs. 7a-7g. 



1839. Graptolites foliaceus, Murch., Silurian System, pi. xxvi. fig. 3. 

 1847. Graptolithus pristis, Hall, Pal. N. York, vol. i. pi. lxxii. 



figs. 1 d to 1 r. 

 1847. Graptolithus secalinus (Eaton), Hall, loc. cit. figs. 2a, b, c. 



