340 BRITISH GRAPTOLITES. 



Var. Daironi, Lap worth. Plate XXXIV, fig. 11. 



1877. Rctiolites pcrlatus, var. Daironi, Lapworth, Grapt. Co. Down, Proc. Belfast Nat. Field Club, 

 p. 136, pi. vi, fig. 30. 



In addition to the typical form Gladiog. perlaius, there occurs at a some- 

 what higher horizon at Craigmichan in S. Scotland a very much larger form 

 designated by Lapworth as B. perlatus, var. Daironi. This form attains a length 

 of at least G cm., and a final breadth of l'l cm. as the result of persistent widening 

 from the proximal end. The thecae are much larger and more distant than in the 

 typical form, numbering twelve to eight in 10 mm. Only one specimen is known, 

 and in this the reticula is hardly discernible, but the general outline of the 

 polypary is well defined in the distal parts. The transverse growth-lines on the 

 floors of the thecae are locally distinct, especially towards the ventral margins, 

 and recall those on the outer walls of the thecae of Petalograptus. They appear 

 to be occasionally fibrous and to be crossed by others in the form of a very fine 

 ribbing. 



Horizon and Locality. — Birkhill Shales (gone of Bastrites maxim its). 



S. Scotland : Craigmichan. 



Collection. — Lapworth. 



Sub-genus Plegmatograptus, nov. 



The second group of the Betiolites includes two species and one variety in 

 which the shape of the thecae is unknown, but which appear to possess a ventral 

 lacinia. As the latter is their most obvious feature we suggest for them the 

 sub-generic name Plegmatograptus. 



Retiolites (Plegmatograptus) nebula, sp. nov. Plate XXXIV, figs. 14 a — d. 



Polypary very short and delicate, about 1 cm. in length, widening rapidly 

 from 1 mm. at the proximal end to a maximum width of 2 mm. Virgella 

 conspicuous, occasionally o mm. in length. Thecae twelve in 10 mm. ; 

 their shape is indistinct, but the processes of their ventral margin give 

 origin to a complete and somewhat complicated lacinia. The elements of 

 the clathria cannot be made out, and the reticula, which is but rarely 

 visible, is usually irregular and ill defined. 

 Description. — P. nebula is a very gregarious form, occurring as faint silvery 

 cloud-like patches upon the surface of the rock. It presents very different 



