388 BRITISH GRAPTOLITES. 



Monograptus argenteus (Nicliolsoii). IMate XXXYIIJ, figs. 5 a — d. 



1860. Graptolltes anjenfens, Nicholson, Ann. Mag. Nat. Hist. [4], vol. iv, p. 239, pi. xi, fig. ID. 



Polypavy 4 — 6 cm. in lengtli, with straight distal ])ortioii and abruptlv recurved 



proximal portion ; ver}^ slender at origin, widening gradually from a small 



and inconspicuous sicula to the l)end, and thence very rapidly till the 



, , ,, , . /TVT- 1 1 X maximum l)readtli of 2'5 mm. is 



Fios. 257 a and h. — Monograptus argenleus (JNicholson). 



attained. Thecal Iouq- narrow 



'O 



'-?^ 



tubes, fifteen to eleven in 10 mm.; 

 in the proximal portion in contact 

 only, with tendency to complete 

 retroversion and isolation of the 

 apertural region ; overlap in- 

 creasing and apertural retro- 

 version decreasing until finally 

 .__,.^ the thocre of the distal portion 



'^^"p--^'^ i'==^ e^^Tr-'' ■' " are simple tubes, overlapping 



fully two-thirds their length, 



a. Proximal portion, showing- sicula. Llanystwimlwy, witll eVCU apei'tui'al margins, 

 near Criccieth ; Llandovery Beds. Coll. Fearnsides. . ■ i r 



h. Proximal portion of polypary, showing form of theca?. VeSCVipf lOU . i lie polypary 111 JMoiiog. 



^^'^ ■ argentens is highly characteristic, with 



its slender straight proximal portion and the rapid change to the robust distal 

 portion after the bend. The whole of the initial portion of the polypary, however, 

 resembles that of M. Ivmatulus in its extreme tenuity. 



The sicula is small and inconspicuous, barely 



Fias. 2r)7 c timl d. — Monograptus , . . , , i n -v j. i j^i 



argentens (Nicholson). reaching 1 lum. lu length, and tailing to reach the 



level of the aperture of th. 1. 

 ^^\ i All the early theca3 are small and inconspicuous, 



M^'^.l apart from the retroverted apertural extremity, 



U'^'^. ij which appears somewhat lobate after compression. 



^u J The retroversion and isolation of the apertural 



^V 1 reG:i(ui of the thecee becomes at first more and more 



pronounced with remoteness from the sicula, and 

 c. Proximal-distal theciE. showin- wlicu the polypary lias attained its maximum 



clianoe from one type to the other. i i:.i j.i ii „ i i.i r i 



EniargementofpartofPi.xxxviii, breadth tlio tlieca3 liavc the appearance of long 

 d.-mLiLc^B. Part of same .pecin.en. tiibcs with a distiiict and isolatc apcrtural hook. 



In the shorter specimens this character may be 

 maintained, with some shortening of the hook, up to the distal termination, but 

 in the longer and more complete specimens it is clear that the retroversion of 



