■MV2 



BRITISH GRAPTOLITES. 



knowledge of the structure of tlie polypary in this genus is still very imperfect, 

 such British examples as have come to hand lead us to infer that the alliances of 

 the genus are rather with the Phyllograptidae and Diplograptidae, and we place it 

 here provisionally in the latter family on account of its biserial character. 



Trigonograptus ensiformis (Hall). Plate XXXV, figs. 1 a — c. 



1865. BetiolUes ensiformis, Hall, Grapt. of Quebec Group, Geol. Survey of Canada, dec. 2, p. 114, 



pi. xiv, figs. 1 — 5. 

 1890. Trigonograptus ensiformis, H. O. Nicholson, Geol. Mag., dec. 3, vol. vii, p. 340, figs. 1, 2. 

 1898. Trigonograptus ensiformis, Elles, Quart. Journ. Geol. Soc, vol. liv, p. 523, fig. 34. 

 1904. Trigonograptus ensiformis, Ruedemann, Grapt. New York, vol. i, p. 727, pi. xvii, figs. 1 — 9. 



Figs. 202 a, b and c. — Trigonograptus 

 ensiformis (Hall). 



Polypary 3—5 cm. or more in length, with an average uniform width of 

 about 5 cm. attained by rapid widening from proximal end, diminishing 

 somewhat distally. Sicula doubtful. Thecae eleven to nine in 10 mm., 



apparently inclined at about 50°, in con- 

 tact throughout their length, their aper- 

 tural edges usually forming collectively 

 an unbroken line constituting the ventral 

 margin of the polypary. Test thick, 

 membranous, marked by growth-lines. 

 Description. — The dimensions of the majo- 

 rity of our British specimens seem to fall a 

 little short of those given by Hall, and examples 

 of about 3 cm. in length are of most frequent 

 occurrence. In other respects, however, our 

 specimens agree well with the American ones. 



In one specimen (PI. XXXV, fig. 1 <z) a 

 membranous structure is seen projecting from 

 the proximal extremity of the polypary. This 

 may in part represent the sicula. 



The thecae present different appearances in 

 different specimens. In some examples (Fig. 

 202 a) the walls of the thecae are parallel and 

 continuous from the ventral edge of the polypary 

 to its central line ; in others (202 b) the walls 

 appear to be curved and there is an oblique ridge 

 running down the centre of each theca. In one 

 Irish specimen (202 c) the polypary appears at 

 first sight to present a totally different aspect on the two sides of the central line. 

 On the one side the thecae resemble those of one of the four stipes of a Phyllo- 



a. Proximal end, showing sicula (?) and mem- 



branous structure. Enlargement of part 

 of PI. XXXV, fig. 1 a. 



b. Distal theca;, showing growth-lines. En- 



largement of part of PI. XXXV, fig. 1 b. 



c. Distal thecae of wide specimen. Eoadside 



N. end of Doolough, Co. Mayo ; Arenig. 

 Coll. Muff and Carruthers. 



