MONOGRAPTUS. 



413 



Associates, etc. — Var. crenulatus is commonly found with Monog. prlodon and 

 M. suhconicus. 



Gollections. — Lapworth and the Authors. 



Moaograptus griestoniensis (Nicol). 



Plate XLI, fiffs. 5 a—d. 



Figs. 279 a—/. — Monograptus (jriesto- 

 niensis, Nicol. 



W 



p 



17 



1850. Graptolites griestoniensh, Nieol, Quart. Jourii. G-eol. Soc, vol. vi, p. 63, fig. 2. 



Polypary several cm. in length, slightly flexuous, widening from a very slender 

 proximal extremity to a maximum diameter of '7 mm. Thecae ten in 



10 mm., with abrupt sigmoid curvature and 

 horizontal apertural margins, situated in dis- 

 tinct excavations occupying fully one-third 

 of the total breadth of the polypary. 

 Description. — The polypary is commonly some- 

 what curved, having the tliecte on the concave 

 margin ; there is, however, some variation in the 

 amount of curvature in the specimens from different 

 localities. Most of those from Grieston Quarry are 

 conspicuously flexuous, while those from the Rhay- 

 ader country tend to be less curved, and those from 

 the Tarannon district are, as a rule, so little curved 

 that fragments appear almost straight and rigid; 

 there would appear to be, however, all degrees of 

 curvature between the extreme types. The poly- 

 pary is extremely slender near the proximal end, 

 not exceeding "2 mm. in width, and the breadth 

 increases very gradually until the maximum of '7 

 mm. is attained. 



The sicula is fairly conspicuous, measuring "6 

 mm. in length and reaching nearly to the level 

 of the aperture of th. 1. 



The thecEe in the maturer parts of the polypary 

 have a length of fully 2 mm. ; they overlap one-half 

 their length or rather less, and the apertural excava- 

 tions are conspicuous, especially in sub-scalariform 

 views. A characteristic aspect presented by the 

 polypary is that in wdiich the apertures are entirely 

 concealed and the stipe has the appearance of a series of vertebrae (Fig. 279/). 

 Affinities, — The extreme tenuity of the polypary and the character of the 



I 



t 



f 



a. Proximal end, showing siciila. Taran- 

 non Kiver, below Dolgau Farm ; 

 Talerddig Grits. Coll. Wood. 



h. Proximal end. Enlargement of part 

 of specimen on same slab as PI. 

 XLI, fig. 5 d. 



c. Distal thecaj ; compressed. Enlarge- 



ment of specimen on same slab as 

 PI. XLI, fig. 5 c. 



d. Distal thecae ; in relief. Afon Cwm 



Calch, nr. Talerddig ; Talerddig 

 Grits. Coll. Wood. 



e. Distal thecse ; compressed. Ibid. 



/■. Theca3 preserved with the dorsal side 

 uppermost, giving characteristic 

 " vertebrae-like " appearance. Same 

 locality as Fig. 279 a. Coll. Wood. 



