460 



BRITISH GRAPTOLITES. 



Gyrtog. rigidus, where it is found with the zone fossil, M. dubius, M. vomerinus and 

 M. llemingii var. -primus'. 

 Collection,— EWes. 



Monograptus dextrorsus, Linnarsson. Plate XL VI, figs. 7 a — d. 



1881. Monograptus dextrorsus, Linnarsson, Geol. Foren. i Stockholm Forhandl., p. 511, pi. xxiii, 

 figs. 1—7. 



Figs. 318a — e. — Monograptus dextrorsus 

 Linnarsson. 





Polypary slender, arcuate or flexed, several cm. in length, of approximately 

 uniform breadth in the distal portion but becoming still more slender 

 towards the proximal end. Thecae seven to eight in 10 mm., on the concave 

 margin ; narrow, with concave outer walls, each theca having the apertural 

 region abruptly folded upon itself to form a peculiar twisted short lobe. 



Description. — The polypary usually shows 

 marked concave curvature, but there is some 

 variation in this respect, some specimens 

 showing a tendency to convexity in the neigh- 

 bourhood of the proximal end. The maximum 

 breadth is about 1 mm. but the extreme 

 proximal portion is very slender. 



The sicula measures 1*4 mm., and extends 

 slightly beyond the aperture of th. 1. 



The theca? are very characteristic, owing 

 to the peculiar character of the apertural lobes 

 resulting from the abrupt folding of the 

 isolated apertural region upon itself, so that 

 abed e the twisted lobe appears to rest as it were upon 



a. slender proximal fragment, Afon Cwm the body or adnate region of the theca ; some- 



Calch, near Talerddig ; Talerddig Beds. 



Coll. Wood. what after the manner of a cup and ball. This, 



b. Distal thecce showing various .appearances. . 



On same slab as pi. xl vi, fig. 7 c. however, is only shown in one aspect of the 



c. Specimen preserved in scalariform view, , , . . . „ . , „ , 



showing apertures. Same locality as Figs, polypary (obverse); it viewed from the reverse 



d. wider' specimen, showing sicuia, probably side often no trace of the twisted lobe can be 



Su!FeamsiS. SpeCieS ' Llanystwmdwy " detected, and the theca seems almost to have a 

 i in profile smooth even edge, which is very deceptive. 



Affinities. — So far as we are aware Monog. 

 dextrorsus stands alone amongst the Monograpti 

 in the possession of these peculiarly shaped theca?, but in the reverse view they may 

 easily be confounded with the theca? of those species which have 'tagged' apertural 

 margin such as J\f. intermedins. The curvature of the polypary, however, is 

 different in almost all cases. 



Belcraig Burn, near Moffat ; Birkhill 

 Shales (Band of Rastrites maximu-i). Coll. 

 Elles. 



