THE ARENIG AND LLANDEILO ROCKS OE ST. DAVII)'s. 651 



towards their distal end ; margin of aperture concave, making 

 an obtuse angle with the axis of the branch. 



There can be little doubt, as Professor Hall has himself stated, 

 that his Graptolithus bryonoides is identical with the Fucoides serra 

 of Brongniart; and our St.-David's and Skiddaw Arenig forms answer- 

 ing to the above description are also referable to the same ; but 

 some specimens figured as Tetragraptus bryonoides seem to be spe- 

 cifically distinct, and should perhaps be referred to the species I have 

 here described as T. Halli. In the absence of sufficiently definite 

 descriptions and sufficiently accurate figures this point cannot be 

 decided ; and we would therefore provisionally place under T. serra 

 all the forms to which the specific name bryonoides has been applied. 



Log. Middle Arenig, "Whitesand Bay. 



Tetragraptus Halli, Hopk., sp. no?. PI. XXXIII. figs. 11 a, 11 b. 



Branches incurved, springing almost immediately from a large 

 pointed sicula, from which they diverge at various angles, at first 

 rapidly, then gradually expanding for the greater part of their 

 length, and again gradually contracting towards their extremi- 

 ties, their extreme width being one fifth of an inch ; hydrothecse 

 from 25 to 30 to the inch, inclined to the axis of the branches 

 at an angle of from 20° to 30°, from five to six times as long as 

 wide, with only their slightly mucronate extremities free, and 

 decidedly curved outwards throughout their entire length ; aper- 

 tural margin concave. 



"We have several specimens, more or less perfect, all of which show 

 one distinctive character — the thecae are scarcely perceptible, appear- 

 ing only as submucronate denticles extending very slightly beyond 

 the general margin of the ventral side of the branches. The curved 

 lines of division can only just be made out in one or two of the 

 better-preserved specimens. Two branches only are usually seen. 



It is possible, as mentioned above, that some of the specimens 

 which have been referred to Tetragraptus serra may be of this species ; 

 but as the two forms are at present only known to occur on the 

 same horizon, this cannot create any confusion. It will be un- 

 necessary to draw a comparison between the two species, as a glance 

 at the figures will at once show their distinctive characters. 



"We name this species after Professor James Hall, to whose clas- 

 sical work on the Graptolites of the Quebec Group we have had so 

 frequently to refer. 



Log. Middle Arenig, Whitesand Bay. 



Tetragraptus Hicksii, Hopk., sp. nov. PI. XXXIII. figs. 

 12a-12d. 



Branches slender, decidedly recurved, from an inch to an inch and a 

 half in length, springing almost immediately from a minute ob- 

 tusely pointed sicula, from which they diverge at various angles ; 

 very gradually expanding near their origin, but more rapidly 



