3-14 



BRITISH GRAT'TOLITES. 



Polypary compact, small and narrow, not exceeding 1*3 cm. in length, with an 



average uniform breadth of about 1 mm. Sicnla obscure, virgella long and 



conspicuous. Thecas fifteen in 10 mm., approaching those of Glyptograptus 



or Amplevoijvajitiix in form, with small overlap; apertnral margins slightly 



oblique, thickened and membranous. Clathria not well differentiated ; 



reticula often conspicuous, meshes extremely minute, usually regular. 



Description. — The polypary is always of small size and very narrow, not 



exceeding 1 mm. at its widest and tapering proximally. A prominent proximal 



Fig. 225.— Gothograptus nassa (Holm), structure corresponding to Wiman's " initial canal " 



(which may be the reticulate covering of the sicnla), 

 is often visible, projecting somewhat beyond the 

 proximal end of the polypary after the manner of 

 the sicnla of Glimacog. Wilsoni and other Diplo- 

 graptidae. The virgella is conspicuous and in one 

 specimen has a length of fully 2 mm. 



The thecas in this species are considered by 

 Holm and other authors to be of the Cli/macograptus 

 type, but as the free edges of the thecas are inclined 

 rather than vertical, and the apertnral margins 

 slightly oblique, they are in form more suggestive 

 of those of Glyptograptus or Amplexograptus. The 

 thickening of the apertnral margin is continued, as 

 in the Swedish examples, in the form of a mem- 

 branous tongue or "lappet" (Wiinan). With the 

 exception of the ventral lattice, none of the ordinary 

 lattices of the clathria are well defined, and the strands and cross-bars are hardly 

 separable from the threads of the reticula, which is here and there crossed sub- 

 regularly by stronger fibres than usual. 



Affinities. — G. nassa differs conspicuously in size and form from all other 

 Retiolites except Gothog. spinosus, from which, however, it can readily be dis- 

 tinguished by the absence of apertnral spines and by the finer meshes of the 

 reticula. 



Horizon and Localities. — Lower Ludlow Shales (zone of Monog. vulgaris). 

 Shropshire: Near Worthen, S. side of Long Mountain; Elton Lane, near 

 Ludlow. N. Wales : Cefn-Gwyn, one mile E. of Eglwys Bach, E. side of Conway 

 Valley. 



Associates, etc. — Gothog. nassa is a rare fossil in Britain; a few specimens only 

 have been found at the localities noted above ; in every case it occurs associated 

 with Monog. vulgaris and M. dubius. 

 Collection. — The Authors. 



Complete specimen, showing tliecse and 

 apertural "lappets." Enlargement 

 of PI. XXXIV, fig. 15 a. 



