TRIGONOGRAPTUS. 303 



g rapt us when shown in profile view, but the edges of the aperture are straight or 

 rounded instead of concave ; on the other side the thecal apertures, or their 

 impressions, are presented in scalariform view. These appearances might be 

 accounted for if the polypary were composed of two stipes disposed at right angles 

 to each other, somewhat like a Phyllograptus with only two of its adjacent stipes 

 developed. Growth-lines appear occasionally on the thecae, but there is no visible 

 reticulation of the test, which in British specimens appears to be membranous and 

 continuous. 



Affinities. — T. ensiformis may be readily separated from its variety lanceolatus 

 by its general shape. 



Horizon and Localities. — Arenig, Upper Skiddaw Slates (Ellergill Beds). 

 Lake District : Mosedale Beck, near Troutbeck ; Ellergill, near Millburn. 

 I n't and: Roadside, N. end of Doolough, Co. Mayo. $. Wales: Pont-y-feni Quarry, 

 3 miles W. of St. Clears. 



Associates, etc. — The associates of T. ensiformis in the Skiddaw Slates are 

 unknown ; but it has been found in Co. Mayo associated with Loganog. Logan i, 

 Phyllog. angustifolius, Tetrag. quadribrachiatus, and Ghssog. acanthus ; and by the 

 officers of the Geological Survey in S. Wales with Didymog. cfr. uniformis. 



Collections. — Nicholson, Geological Survey of England and Wales, Sedg- 

 wick Museum, Muff and Carruthers. 



Var. lanceolatus (Nicholson). Plate XXXV, fig. 2. 



1869. Trigonograjpsus lanceolatus, Nicholson, Ami. Mag. Nat. Hist. [4], vol. iv, p. 232, pi. xi, fig. 6. 



A variety of Trigonog. ensiformis is also found in the Skiddaw Slates and 

 „ onQ „ . , ., . has been described by Nicholson. It differs chiefly 



Fig. 203. — Trigonograptus ensiformis, J J 



var. lanceolatus (Nich.). from the typical species in the rapidity with which 



it widens in the proximal region, and in having a 

 distinct zigzag septal groove or thickening. The 

 only British specimen known to us shews no details 

 as to the structure of the thecge, but there are 



V fourteen in 10 mm. 

 Distal part showing position of thecsB Horizon and Locality. — Upper Skiddaw Slates 



and zigzag septal groove; pre- /T?lWmll ttp/M 



served as a east. Enlargement of (^lieiglll DeUSJ. 



part of pi. xxxv, fig. 2. ^^ jj^.^ . Ellergill, Millburn. 



Associates, etc. — Unknown. 



Collection. — British Museum (Natural History). 



