THE AKENIG AND LLANDEILO KOCKS OF ST. DAYID's. 657 



1850. Diprion foliaceus, Harkness, Quart. Journ. Geol. Soc. vol. vii. 

 p. 64, pi. i. figs. 13a, b, c. 



1852. Diplograpsus foliaceus, Geinitz, Graptolithen, taf. i. figs. 29, 30. 



1859. Graptolithus pristis, Hall, Pal. New York, vol. iii. Supple- 

 ment, p. 516. 



1866. Diplograpsus barbatulus, Salter, Mem. Geol. Surv. vol. iii. 

 pi. xi.A. figs, le, d. 



1868. Diplograpsus pristis, Carruthers, Geol. Mag. vol. v. pi. v. 



figs. 13 a, b, c, d. 



1869. Diplograpsus pristis, Hopk., Journal Quekett Micros. Club, 



vol. i. pi. viii. fig. 11a. 

 1872. Diplograpsus pristis, Nicholson, Monograph British Grapto- 

 litidae, pt. i. figs. 22, 26, 50. 



Length from one to three inches, average breadth about one sixth 

 of an inch, margins parallel distally, converging towards the 

 proximal extremity, which is normally furnished with a radicle 

 and two marginal spurs; virgula very distinct, greatly pro- 

 longed distally ; hydrothecse 18 to 30 to the inch, making a 

 very small angle with the general axis of the polypary ; aper- 

 tural margin oblique; apertural spine single, frequently 

 wanting. 



It is almost impossible to frame a diagnosis sufficiently broad to 

 include the extreme varieties of this protean form. The foregoing 

 description embodies all the more constant characteristics of the 

 species. The features which seem to be least variable in British 

 examples are the great length of the distal prolongation of the 

 virgula, and the very small angle included between the outer 

 margin of the theca and the general line of the direct septum. 

 The former is never less than half the length of the polypary, and 

 the latter oscillates between 15° and 20°. The specimens we 

 have collected at Meadowtown, Shropshire (the locality which 

 afforded the examples on which this species was originally founded), 

 show a very stunted variety, which rarely exceeds an inch in length, 

 and bears above thirty hydrothecse to the inch. There is little room 

 for doubting its identity with the form described and figured by 

 Murchison and Sowerby as Graptolithus foliaceus. The same form, 

 together with D. secalinus and several other varieties much further 

 removed in general aspect from the typical D. foliaceus, occur in 

 association in South Scotland and elsewhere, and are all connected 

 together by intermediate forms. 



The variety which occurs at Abereiddy Bay appears to be 

 identical with the Graptolithus secalinus of Eaton, as figured by 

 Professor Hall in his ' Palaeontology of New York,' vol. i. pi. 1. 

 figs. 2 a, b, c. It differs from the majority of the^later varieties in 

 the marked parallelism of the greater portion of the ventral mar- 

 gins, and in the compact arrangement of the thecse, which are 

 closely appressed to the periderm, the denticles projecting very 

 slightly beyond the general boundary of the fossil. In our ex- 



2x2 



