MONOGRAPTUS. 389 



the apertural region gradually diminishes, and finally the thecas are simple tubes, 

 with even and horizontal or slightly everted apertural margins. The mature 

 thecffi are very slender and a horizontal line drawn across the polypary in its distal 

 portion would cut four different thecoe. 



Affinities. — As respects its general form, Monor/. argentens approaches 

 M. limatulus, but distally it is a far larger and more robust species, and the thecal 

 are different in form. 



Horizon and Local Itics.—Lhmdoycvy (Skelgili Beds), sub-zone of M. argoiteus. 



Lal-c Bldrict : Skelgill ; Mealy Gill, etc. N. Wales: Llanystwmdwy, near 

 Criccieth. 



Associates, etc. — Monog. argenteus occurs in abundance in a definite band in the 

 Stockdale Shales associated with Monog. leptotheca, M. grcgavias, M. communis, 

 Glijptog. sinaatus, etc.; and with similar associates at a corresponding horizon in 

 N. Wales. 



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

 Marr, Lapworth, and the Authors. 



Var. cygneus (Tornquist). Plate XXXVIil, figs. 6 a — d. 



1892. Monograplus cygneus, Tornquist, Luuds Uuiv. Arssk., vol. xxviii, p. 1(.>, pi. i, figs. 28 — 31. 



While the typical Monog. argenteus has only as yet been recognised at 

 Llanystwmdwy and in the Lake District, there occurs fairly abundantly in other 



localities, both at the same horizon and rangiug 



Figs. 2.58 a and h. — Mono(iniptus argen- i • i ,. ^ • -i 



tens, vav. cygneus, Tih-nqwist. somcwhat higher, a lomi whicli presents many 



1^ / -_^ points of resemblance to it, but is altogether 



I // narrower. This seems to come very close to Torn- 



^'l'?i quist's M. cijgneus, though, as originally described 



I /K Wu by him, his examples were shorter than is the case 



/' F y with many of our British specimens. 



h f J'M In this form, which we consider best regarded as 



i^ Lt-i) a variety of M. argenteus, the theci\3, Avhich are of 



' ji^ |,/>^ the same general character as those of M. argeuteus, 



' ' ^-^ are inclined at a lower angle, and are more distant 



" ^ (eight in 10 mm.) than those of the typical form : the 



a. Proximal typo of theca3. Enlarge- ^ <j ' . 



iiieut of part of PI. xxxviii, overlap does not exceed one-half the thecal length, 



fig. G c. 



h. Distal type of tht'cse. Part of same and the polypary has an average breadth in the 



specimen. i- i ■ i • -i mi i i 



distal portion not exceeding To mm. The thread- 

 like proximal portion is rather more than 1 cm. in length, and has precisely the 

 same type of theca3 as has the typical species. 



