120 



BRITISH GRAPTOLITES. 



Figs. 2S2 c, d. — Monographis priodon 

 (Bronn). 



mode of origin of the successive theca?, and hence the amount of overlap, is clearly 

 visible, the theca? appearing as more or less sacculate tubes diminishing towards 



their apertural regions, which are reflexed into a 

 robust hook. In the obverse aspect, however, the 

 place of origin of the theca? is not seen, and hence 

 the theca? appear to be shorter and broader, in con- 

 tact only, and with narrower reflexed apertural 

 regions ; thus in this aspect extra prominence is 

 given to the upper wall of each theca. 



Affinities. — Monog. priodon resembles M. pandus 

 and M. Marri more than any of the other members 

 of the group, so closely indeed as to have led to 

 great confusion. From M. pandus, which it also 

 resembles in general form and size, M. priodon 

 differs in the characters of the thecas, which have 

 (1) a relatively longer hook, (2) are inclined at a 

 lower angle, (3) have more overlap, and (4) are 

 more closely set in the proximal region than those 

 of M. pandus. In M. Marri the polypary is more 

 slender than in M. priodon, and there is also rela- 

 tively less isolation and retroversion of the apertural 

 region of the theca?. 



Horizon and Localities. — Gala-Tarannon to Lower 

 Wenlock Shales (zone of M. crispus to zone of M. riccartonensis). 



S. Scotland. — Grieston ; Tushielaw ; Selkirk; Galashiels; Riccarton, etc. 

 Lalce District. — Long Sleddale; Nanny Lane, Troutbeck; W. of Bluecaster 

 Side, Sedbergh, etc. Harter Fell ; Wandale Hill; Bluecaster; Middle and Near 

 Gills, Cautley. Wales. — Pencerrig, Builth ; Trecoed, Builth ; Walcot Quarry, 

 Chirbury ; Penarth Quarry, Carrog ; Tarannon River ; Talerddig, etc. 



Associates, etc. — Monog. priodon is one of the commonest and one of the best 

 known of all Graptolites. It seems to make its first appearance in the Tarannon 

 Shales in the zone of M. crispus, and occurs there in association with M. crispus, M. 

 discus, and M. Marri ; it is, however, at that horizon subordinate in numerical 

 importance to M. Marri and M. pandus. It increases in numbers in the higher 

 beds until in the zone of Oyrtog. Murchisoni at the base of the Wenlock Shales, it is 

 one of the commonest Graptolites as well as one of the most characteristic of that 

 horizon. Its vertical range extends also into the succeeding zone of M. riccar- 

 fmiensis, but thereafter its place is taken by M. Flemingii. 



Collections. — British Museum (Natural History), Sedgwick Museum, Belfast 

 Natural History Museum, Geological Survey of England and Wales, and of 

 Scotland, Lapworth, Fearnsides, and the Authors. 



c. Distal thecse (reverse aspect) on 



same slab as Fig. 282 a. Apertures 

 of thecee partially embedded in the 

 limestone. 



d. Distal thecse in low relief, showing 



apertural margins, and short inter- 

 thecal walls, obverse aspect. Tar- 

 annon River ; Talerddig Beds. Coll. 

 Wood. 



