MONOGRAPTUS. 



363 



Figs. 236 o — c. — Monograptus cyphus, Lapw. 



between tlie apertures of tli. 1 and tli. 2, and the first tlieca (tli. 1) arises about 

 one-third of the way along its length. 



All the thecse are simple, 

 straight - walled, narrow 

 tubes, with normal aper- 

 tural margins which may 

 appear slightly concave or 

 even convex after com- 

 pression ; in their general 

 characters they resemble 

 those previously described 

 as characteristic of the 

 Dichograptidse ; they have 

 a length of 3 mm., and are 

 aljout six times as long as 

 wide. The amount of over- 

 lap increases steadily from 

 one-third of their length 

 near the sicula to one-half 

 in the broadly recurved portion, and eventually to two-thirds of the total length 

 in the mature portion of the polypary, where a horizontal line drawn through 



an aperture would cut three different theca). The 

 apertural margins in the mature portion lie oblicjuely 

 with respect to the general direction of the poly- 

 pary, but are in every case normal to the theca 

 itself. In specimens preserved in relief the overlap 

 of the thecal is clearly seen, but occasionally, as 

 the I'esult of compression, the tlieca appear some- 

 what like those characteristic of Cliniacugrapfas, and 

 the overlap is then apparently less (PL XXXVI, 

 fig. 1 a). 



Abilities. — Moiiog. ci/plius resembles in its general 

 form several other members of the group ; it may, 

 however, be readily distinguished by the breadth 

 of the distal portion of the polypary, and the simple 

 character and extensive overlap of its thecas. 

 Distal fragments may resemble fragments of Monog. 

 leptoUieca , but the polypary of M. ci/phas is more 

 curved, and the overlap of the thecEe less for a given breadth of stipe. 



Horizon and Localities. — Llandovery (Birkhill Shales), zone of J/, cijphus. 



8. Scotland: Waterfall, Long Linn, Doblj's Linn, Moffat; Garple Linn, Moffat; 



a. Proximal end, sliowincf sicula. Rht'idol Gorge, Cardiganshire ; Ivlieidol 



Group. Coll. O. T. Jones. 

 6. Ditto, in full relief. Same locality. 



c. Proximal end, .showing abrupt curvature. Garple Linn, Moffat; Lower 

 Birkhill Shales. Coll. Geol. Survey of Scotland. 



Figs. 236 d and e. — Monograptus cyphus, 

 Lapw, 



M 



d 



d. Theca; near the proximal end of the 



polypary. Dobb's Linn ; Lower 

 Birkhill Shales. Coll. Lapworth. 



e. Distal theese of same specimen. 



