DIDYMOGRAPTUS. 16 



Didymograptus hirundo, Salter. Plate I, figs. 5 a—c. 



1863. Didymograpsus hirundo, Salter, Quart. Journ. Geol. Soc.,vol. xix, p. 137, fig. 13/. 



1866. Didymograpsus hirundo, Salter, Mem. Geol. Survey, vol. iii, p. 331, pi. ii, figs. 6 and 7. 



1868. Didymograpsus patulus, Nicholson, Quart. Journ. Geol. Soc, vol. xxiv, p. 135. 



1870. Didymograpsus patulus, Nicholson (pars), Ann. Mag. Nat. Hist. [4], vol. v, p. 340, pi. vii, 



fig. 1 a. 

 1898. Didymograptus patuhis, EUes, Quart. Journ. Geol. Soc, vol. liv, p. 504, figs. 22, 23. 



O 



1901. Didymograptus patulus, Tiirnquist, Lunds Univ. Arssk., Bd. xxxvii, Af. 2, Nr. 5, p. 15, pi. ii, 

 figs. 1 — 6. 



Stipes several centimetres in length, Inroad even at orig-in,and attaining a maximum 



width of 4 mm., diverging from a conspicuous sicula at an angle of 180°, and 



continuing to grow in a horizontal direction. Thecae nine to ten in 10 mm., 



inclined at various large angles in different parts of the stipes, three times 



as long as wide, free about one fourth their length. Apertural margins 



normal, concave, and submucronate. 



Description. — In this species, though the stipes grow throughout their length 



in an approximately straight line, there is a tendency to upward curvature at the 



distal extremities in some of the larger specimens. At times the stipes seem to 



have attained an enormous length ; a slab in the Keswick Museum of Local 



Natural History shows a large specimen, one stipe of which is complete and 



measures 30 '5 cm. in length ; the total length of this specimen must therefore 



have been 61 cm. Another specimen in the Woodwardian Museum, Cambridge, 



must have measured 53 cm. when complete. So far as we are aware these are the 



longest Didymograpti on record. The stipes are also characteristically wide, the 



average breadth being 2'5 to 2*1 mm. ; but in the larger specimens referred to above 



the width may be as much as 4 mm. The variation in the width of the stipe is due no 



doubt in part to the variation in the length of the thecoe and their inclination : in 



some young specimens, therefore, the maximum width is never attained. As a 



general rule the stipes are wide at their origin, and continue to broaden slightly 



toward their distal ends, but a slight diminution in width takes place immediately 



before the distal extremity is reached. 



The sicula is unusually long (3*2 rnm.), but owing to the size of the polypary it 

 does not appear remarkable. It is narrow, especially in the apical part; at its 

 aperture it is about "o mm. in width. The first theca (tli. V) seems to originate 

 very near the apex of the sicula ; it grows at first closely appressed to the sicula in 

 a straight downward direction, but then curves away, leaving the sicula free near its 

 aperture for about one third of its length on the left side. The crossing canal 

 originates near the apex of the sicula, to Avhich the second theca (th. 1") is closely 

 appressed. Theca 1^ does not grow much farther down than theca 1^ consequently 



