MONOGKAPTUS. 



149 



Figs. 308 « and b.—Monograptus lobiferus 

 (M'Coy.). 



\ 



« 



i 

 * 



4 



a. Proximal end, showing sicula. En- 



largement of part of PI. XLV, fig. 

 1 e. 



b. More distal portion of same specimen, 



in full relief. 



Pigs. 308 < 



-e. — Monograptus lobifei us 

 (M'Coy). 



ments 8 cm. or more in extent are known, and they 

 exhibit every degree of flexure. Sometimes there is 

 conspicuous curvature, with the thecne on the 

 concave margin, at others slight irregular flexure, 

 the polypary being at first concavely and then 

 convexly curved. Occasionally long fragments are 

 met with which appear to be so slightly flexed that 

 they are practically straight. The proximal end is 

 slightly recurved, but the curvature only involves a 

 few of the earlier thecas. 



The sicula is small, not exceeding 1-2 mm. in 

 length, and th. 1 originates near its base and 

 terminates a little distance below its apex. 



The thecas throughout are of one general type. 

 The characteristic feature is the prominence of the 

 lobe, the round or disc-like appearance of which is 

 clearly due to the retroversion and torsion of the 

 whole apertural region. This is coiled upon itself 

 in such a manner that, after compression, the plane 

 of the aperture almost faces the general line of the 

 polypary, but is slightly twisted, so that the actual 

 thecal aperture is seen in one aspect only (the 

 obverse), where it practically faces the observer. 

 In this aspect the roundness of the lobe is accen- 

 tuated by foreshortening. In the other aspect (the 

 reverse), where no aperture at all is visible, the lobe 

 is turned away and the adnate region of the theca 

 is relatively more conspicuous. 



Affinities. — There appears to have been some 

 confusion in the minds of palaeontologists between 

 Munog. lobiferus and its ally M. millepeda, both of 

 which were amongst the earliest graptolites to be 

 described. M. millejpeda is a form which is in reality 

 quite distinct from M. lobiferus, but the name of the 

 latter species often appears as a synonym for that 

 of the former. This seems to be due to the fact 

 that the strongly recurved proximal end of M. 

 millepeda has been erroneously figured as belonging to M. lobiferus (Lapworth, 

 Geol. Mag., 1876, pi. xx, fig. 1, c, d, e), whereas the proximal end of M. lobiferus 

 is but slightly recurved and involves but few thecae. This mistake led Tornquist to 

 describe as distinct a third species, viz. M. Jiarpago, in which the proximal end was 





_4^ 



/<? 



c d e 



Proximal fragment, preserved partly 

 in relief, partly as a cast. Llany- 

 stwmdwy, near Criccietli. Coll. 

 Fearnsides. 



Distal thecae in relief, showing some 

 of the apertures partly facing and 

 others partly turned away from the 

 observer. Skelgill Beck, Amble- 

 side; Skelgill Beds. Coll. Sedg- 

 wick Museum. 



Compressed thecae, reverse aspect. 

 Enlargement of part of counter- 

 impression of PI. XLV, fig. 1/. 



