16 



BRITISH GRAPTOLITES. 



Fia. 9 a. — Didymograptus hirvMdo, Salter. 



Proximal end, obverse view. Enlarge- 

 ment of PI. I, fig-. 5a. 



Fia. 9 b. — Didymograptus hirundo, Salter. 



the proximal part of tlie polypary in this species has an unusually symmetrical 

 appearance. In the obverse view the sicula is seen to be somewhat obliquely 



placed, but this is not so apparent in the reverse 

 view, where the right side and part of the aperture 

 are concealed by the growth of theca 1^. 



The thecse number generally about nine to ten 

 in 10 mm.; but ten to eleven in 10 mm. may be 

 counted near the proximal end. In none of the 

 specimens that have come under our notice have we 

 found the number greater than eleven, and that 

 number is only attained when the thecse are imperfectly developed, and never 

 in the distal portions of the stipe. Nicholson, however, has given twelve to fourteen 



in 10 mm. for some forms from the Skiddaw Slates, 

 which he refers to D. patulns. Most of the speci- 

 mens in his collection referred by him to Hall's 

 species should, we think, be referred to Salter's D. 

 hirundo, except those figured by him with closely 

 set thecse or "wath concavely curved dorsal walls. 

 These in our opinion are more closely related to B. 

 nitidus. Each mature theca measures fully 3"2 mm. 

 The inclination of the thecae to the axis of the stipe varies in different parts : near the 

 proximal end they are inchned at about 45° ; a little further away from the sicula 

 they curve in such a manner that, while the inclination at their bases is only 25°, 



the inclination in the apertural region makes an 

 angle of G0° with the general direction of the stipe. 

 In certain specimens it appears that communication 

 between the different thecse was not so perfectly free 

 as in some forms, but was effected through a more 

 constricted aperture at the base of each cell. It is 

 characteristic of the species that a vertical line 

 drawn through the aperture cuts the base of the 

 The tlieca3 are sometimes so well preserved that they 



Proximal end, reverse view, 

 largement of PI. I, fig. 5 h. 



En- 



Fia. 9 c. — Didymograptus hirvndo, Salter. 



Distal theca', preserved as a cast, 

 showing apparent constriction of 

 the common canal. Randal Crag, 

 Skiddaw. Coll. Woodwardian Mu- 

 seum. 



second tlieca in advance. 



show lines of growth parallel to their apertures. 



Affinities. — This species presents many points in common with V. patulus and 

 D. sparsus, — in fact, some distal fragments can hardly be distinguished from Hall's 

 species ; and for this reason it was formerly included under the head of D. patuluf, 

 and Salter's specific name fell into abeyance. But since the characters of the 

 proximal end of Hall's species are now more fully known, it is clearly recognisable 

 as a distinct species, and Salter's name must therefore be re^aved. D. hirundo 

 differs from D. patulus in its proximal end, which is " closed," in contrast with the 

 " open " character of the initial region of 1). patulus. The distal fragments of the 



