THE ARENIG AND LLANDEILO ROCKS OF ST. DAVID'S. 647 



gent. Most palaeontologists hitherto have somewhat dubiously 

 divided them between the two species Didymograptus MurcTii- 

 soni, Beck, and D. geminus, His. There are, however, in reality 

 not two, but four, tolerably distinct forms in this group, which 

 embraces the D. hifidus, D. indentus (with var. nanus), D. Murclii- 

 soni, and D. furcillatus of the present memoir ; and if D. Mur- 

 chisoni and D. geminus are to be regarded as distinct species, the 

 remaining types must also receive distinctive titles. The forms 

 belonging to this group are certainly most intimately allied; and 

 specimens are sometimes met with which point to a gradual trans- 

 ition between some of them ; and the remaining blanks are almost 

 filled in by foreign forms. It may consequently be found necessary 

 at some future date to group them all in a single highly variable 

 species. As each form of the group, however, can be easily recog- 

 nized at a glance, and is characteristic of a definite portion of the 

 succession in the St.-David's rocks and elsewhere, they may most 

 conveniently be regarded in the meantime as distinct but closely 

 allied species. 



Hisinger's figure of Prionotus geminus (Lethsea Suecica, Supp., ii. 

 tab. ii. fig. 3) shows us a diminutive form bearing wide branches 

 with close-set denticles, and having a prominent sicula and arch- 

 like axil. It approaches closest in its general features to the pre- 

 sent species, but does not seem to be absolutely identical. The 

 Graptolithus geminus of Scharenberg (Ueber Graptolithen, taf. i. 

 figs. 1, 3, 4) is a very different species, and more nearly resembles 

 our Didymograptus furcillatus. 



Loc. Upper Arenig, Porth Hayog, Ramsey Island. 



DlEYMOGRAr-TUS INDENTUS, Hall, Sp. PI. XXXIII. figs. 7 0,-7 C 



1858. Graptolithus indentus, Hall, Rep. Geol. Surv. Canada for 



1857, p. 128. 

 1865. Graptolithus indentus, Hall, Grapt. Quebec Group, p. 74, pi. i. 

 fig. 20. 



/3. 

 Var. nanus, Lapw. PI. XXXIII. fig. 7 d ; PL XXXY. figs. 

 4 «-4 c. 

 1868. Didymograpsus geminus, Nich. (non His.), Quart. Journ. 



Geol. Soc. vol. xxiv. p. 124, pi. v. figs. 8, 9. 



1870. Didymograpsus geminus, Mch. Ann. & Mag. Nat. Hist. 



ser. 4, vol. v. p. 346, fig. 6. 



Branches slender, maintaining an almost equal width throughout, 



from one to two inches (or more) in length, diverging from 



each other at a primary angle of about 300 degrees, gradually 



incurving for the first quarter of an inch, and nearly parallel 



for the remainder of their length; sicula slender, acutely 



pointed; virgula very distinct; hydrothecse from 20 to 25 



to the inch, inclined to the axis at an angle of from 30 to 35 



