154 



BRITISH GRAPTOLITES. 



Figs. 9G a and b. — Dicellograptus sex- 

 tans, Hall. 



detected ; in the reverse aspect of the polypary it seems to be entirely concealed 

 by the growth of the earliest thecae, though in the obverse aspect it is more or less 



clearly visible throughout its length in well-preserved 

 specimens. The earliest thecae, th. I 1 and th. I 2 , are 

 of considerable size, and unlike the corresponding 

 thecae of most of the other Dicellograpti, only a small 

 fraction of their length assumes a horizontal direc- 

 tion, most of their growth being obliquely upward 

 and outward. The same is also the case with th. 2 1 

 and th. 2 2 , which, however, develop alternately as 

 in other species. Hence the proximal end, owing to 

 this alteration in the direction of growth, approxi- 

 mates (like D. anceps) closely to what may be termed 

 the Diplograptid type. 



The thecae subsequently developed measure only 

 1 mm. in length, and on the free part of their outer 

 walls short projecting spines may frequently be 

 detected, especially on the more proximal thecae. 

 These are, however, only visible in the better pre- 

 served specimens, and good examples of this species 

 are very rare. 



Affinities. — The direction of growth of the 

 earliest thecae, and hence the " Diplograptid " appearance of the proximal end, 

 brings D. sextans into close relationship with the Dicranograpti. This was probably 

 the reason why many earlier observers referred it to that genus. This form of 

 the proximal end, however, is practically identical with that of some other species 

 of Dicellograptus, as, for example, D. anceps, and varies so little from the more 

 ordinary type exemplified in D. divaricatus, etc., that there is no reason to exclude 

 D. sextans from the genus Dicellograptus on this account. It may, however, be 

 regarded as one of the intermediate forms connecting this genus with that of 

 Uicranograptus. Hopkinson separated from D. sextans (Hall) as a distinct species 

 a form, which he called Dicranog. formosus, as having " a longer stem, a smaller 

 angle of divergence, longer and more robust stipes and more elegantly curved 

 thecae." A comparison, however, of a large number of specimens seems to show 

 the existence of every variation between this and the typical form, and we therefore 

 consider it best to include both under the one name of Dicellog. sextans. 



D. sextans can be readily distinguished from other Dicellograpti by its angle of 

 divergence and by its close-set thecae. 



Horizon and Localities. — Llandeilo, Glenkiln Shales. 



Radnorshire : Builth Road. Wales ; Tiddyndicwm. S. Scotland — Moffat Area : 

 Glenkiln Burn; Belcraig Burn ; Hawkwood Burn ; B erry bush Burn ; Slate Burn ; 



a. Proximal end, showing initial and 



lateral spines and typical form of 

 axil. Enlargement of part of PI. 

 XXI, fig. 1 d. 



b. Form of axil approximating to that 



of a Dicranog raptus. Enlargement 

 of part of PI. XXI, fig. 1 a. 



