Vol. 56.] FORMATION AND ITS GRAPTOLITB-FATJNA. 485 



2. Genus Retiolites. 

 Retiolites spinosus, sp. nov. (PI. XXV, figs. 29 a & 29 b.) 



Polypary diprionidian, straight, from 12*7 to 19 mm. (*5 to -75 

 inch) long. Virgula straight, produced beyond the distal end. 

 Width at proximal extremity = about *89 mm. (-035 inch), in- 

 creasing gradually and uniformly to a maximum diameter of 

 1*78 mm. (-07 inch), exclusive of thecal spines. 



Proximal Extremity. — I have not been able to determine 

 with certainty the details of the proximal extremity. The aperture 



of the first theca is -6 mm. (-024 inch) 



■p- 2ft Retiolites sui- above the base, so that the initial canal, 



nosus, sp. nov., from if existent, must be very short. There 

 Vicarage Road, Builth appears, however, to be a true sicula 

 s $\ with a continuous periderm -5 mm. 



(•03 inch) long, extending halfway 

 between the first and second thecae, 

 and continuous with the virgula (text- 

 fig. 26 a). 



Thecae. — Twenty-eight to twenty- 

 six in 1 inch (eleven to ten in 1 cm.). 

 The apertures appear to be at right 

 angles to the axis of the polypary, 

 and are each provided with one or 

 two long spines, 1*01 mm. (*04 inch) 

 and more in length, which are gene- 

 rally straight and directed at right 

 angles to the polypary. In the British 

 specimens one spine alone is usually 

 visible, arising from the aperture of 



a=Proximal extremity. each theca '■> but in Bohemian specimens 



5=Enlargement of fig. 29 a collected from Borek the polypary has 

 in PL XXV. not been compressed quite symmetric- 



ally, and there are two spines visible. 

 Consequently it is probable that there are two in the English forms. 

 It is difficult, in the compressed state of the specimens, to make out 

 the details of the network. Main threads would seem to arise from 

 the virgula and outline the apertures of the thecae. The outer 

 walls between the apertures are angular and concave, giving to 

 the outer edge of the polypary a zigzag appearance. Straight 

 threads at right angles to the polypary connect these angles with 

 the virgula. The details of the network, however, will be best 

 seen from the figures. 



Affinities. — Retiolites spinosus may be readily distinguished 

 from all other species of the genus by (a) the general shape of the 

 polypary, and (6) the long thecal spines. It is one of the most 

 characteristic forms that occur in the zone of Monograptus Nilssoni, 

 especially in the Builth district, where it is found covering large 



