Vol. 54.] FAUNA OP THE SKIDDAW SLATES. 518 



Genus Azygograptus, Nicli. & Lapw. 



1875. Nich. Ann. Mag. Nat. Hist. ser. 4, vol. xvi, p. 269. 



The original description of the genus, which seems to have been 

 founded on the characters of Azygograptus Lapwortlii^ Wich., re- 

 quires some modification when the characters of the other species 

 are taken into account. It can no longer be said that the stipe ' is 

 developed from the central portion of the sicula on one side,' and 

 hence the relationship to the Nemagraptidae is no longer obvious. 

 For my own part, I much prefer to regard it as belonging to the 

 Dichograptidae, since 1 consider it most closely related to the forms 

 in that family. It has no connecting-canal, and, therefore, not 

 more than one stipe can arise, but otherwise it closely resembles a 

 Didymograjptus. Holm has described a form which he believes to 

 be intermediate hQtweeTithe Didymograpti and the Azygogrctpti ; in 

 this one stipe is developed and also a connecting-canal, from which, 

 however, no second stipe arises. If this be the case, the Azygo- 

 grapti are undeniably linked to the Didymograpti, and should, in. 

 my opinion, be included in the same family. 



A modified definition oi Azygograptus might be stated as follows : — 

 ' Rhabdosoma single, unilateral, consisting of a single stipe with 

 cells on one side only. This stipe originates from the sicula at 

 various levels.' 



(a) AzYGOGKAPTiJS Lapworthi, Nich. 



1875. Azygograptus LajpwortJii, Nich. Ann. Mag. Nat. Hist. ser. 4, vol. xvi, 

 p. 269 & pi. vii, figs. 2-2 c. 



Rhabdosoma consisting of a single curved stipe with cells on one 

 side only. This stipe originates at a point rather more than 

 half way down the sicula. The stipe makes with the sicula an angle 

 of 110°. 



The cells number twenty in 1 inch (eight in 10 mm.) ; they are long 

 and narrow, and the width at the apertare is about twice as great 

 as at the origin. Outer walls straight, or with very slight concave 

 curvature in some of the earliest developed thecee. Inclination 20^ : 

 the cells are in contact for a very short distance at their base. 

 Cell-apertures straight, perpendicular to the general direction of the 

 stipe. 



The sicula is large and conspicuous when present, but many of 

 the stipes seem to be broken off from the sicula at the point of 

 connexion. It is -g-V inch (1*26 mm.) long (Nicholson gives -^^ inch 

 = 1*05 mm. as the length, but all the specimens that I have seen 

 are slightly longer), and is always completely free at its aper- 

 tural end for fully one-third of its length ; the width at the 

 aperture is about -^^ inch ('5 mm.). The apical part of the sicula 

 occasionally shows transverse rings. 



The stipe is very slender at its origin, the width opposite the 

 aperture of the first theca not exceeding J-g- inch (about '5 mm.) 



2n2 



