410 



BRITISH GRAPTOLITES. 



Siciila conspicuous. ThecaB short, eleven to ten in 10 mm. Avitli slight ogee- 

 form curvatiu-e, overlapping about one-half their length, and having their 

 apertural margins either everted or appearing in distinct excavations. 

 Description. — The polypary varies very much in size, but all fragments of the 

 initial portion show the characteristic rapid Avidening, the maximum breadth being 

 attained within 2-5 cm. of the proximal end. 



The conspicuous sicula has a length of fully 1"5 mm. and reaches well above the 

 level of the aperture of th. 1 ; its outer wall is slightly curved, but the inner 



continues the straierht line of the 



Figs. 275 a — •/. — Monogvapiiis vomerinns, Nicholson. 



■/ 





M 



a. Proximal end, sliowin" 



dorsal wall of the polypary. 



All the thecfB are short, the distal 

 ones having a length of only 1'5 mm. 

 and the proximal ones are much shorter 

 with the exception of th. 1 , Avhich has 

 a length of 1 mm. The thecae present 

 various appearances according to the 

 different conditions of preservation. 

 Sometimes they seem to have abrupt 

 sigmoid curvature and the apertural 



f.„ ., ,, V* F-,- » margins are situated within distinct 



ffl [^ ^^ ^*^ excavations occupying about one-third 



of the total breadth of the polypary 

 (Fig. 275./'). This is their most 

 characteristic aspect. The theca3 of 

 specimens preserved in relief, how- 

 ever, show a tendency to the ogee 

 curvature characteristic of M. jirioduu 

 (Fig. 275 c), and in this case their 

 apertures are either very conspicuous 

 (Fig. 275 c) or wholly concealed (Fig. 

 275 rf). 



In the most characteristic view the 

 thecas obviously recall those of Climacogra^tm^, but it is clear that the tlieca3 cannot 

 have been quite so simple in form, and there must have been some torsion of the 

 axis of the tliecge in order to account for the different appearances after compres- 

 sion (Figs. 275 ft—/). 



Affinities. — Monorj. vomeriiius is very closely related to all its varieties, and from 

 most of them it is indistinguishable in the distal portion. The proximal ends are, 

 however, distinct, and in var. crejiulatas the thecal apertures are narrower than 

 in the typical form, while in var. hasiliciis the polypary is altogether more roljust. 

 Horizon and Localities.— Wenlock Shales (all zones). 



d e f 



^, sicula ; preservi'd as a cast. 

 Euthin Road, nr. Llangollen ; Wenlock Shales (zone of 

 Cijrtog. MHrchiso7n). Coll. Elles. 

 h. Proximal end, showing sicula, preserved in relief. 

 Lledcwm Stream, New ydd Fynyddog, nr. Llanbrynmair ; 

 Wenlock Shales. Coll. Wood. 



c. Theca; near proximal end ; in full relief, showing torsion 



of thecal axis. Ibid. 



d. Adult theCEB. seen partly from the back so that the 



apertures are concealed. Enlargement of part of PI. 



XLI, fig. 1 e. 

 c. Adult tliecffi with apertures facing the observer. Reverse 



side of Fig. 276 d. 

 f. Adult theca; as they usually appear in a compressed 



specimen. Pencerrig, nr. Builth ; Wenlock Shales 



(zone of Cijrtog. Murchisoni). Coll. Elles. 



