MONOGRAPTUS. 403 



Gkoup II. 



Mijiioijnqjti in wliicli the mature thecal are tubes with fiowiiig siguioid 

 curvature and oblique apertural margins. 



The Graptolites included in this group vary a good deal in the amount of 

 sigmoid curvature of their thecal walls. Most of the species typically possess 

 tliecae resembling those of Liytogrcqjfas ; but the group includes at the one 

 extreme M. atavus, a form in which the thecae are not very much curved, and at 

 the other extreme M. argutiis, in which the thecee have marked curvature and 

 both introversion and introtorsion of the apertural margin, characters approxi- 

 mating to those typical of the DiceUograpfl. 



The group includes the following species : Mo nog rapt (is ataints, M. Saudersoni, 

 M. iitcoDiiiiodas, M. tenuis, M. argatus. 



Monograptus atavus, Jones. Plate XXXIX, figs. 1 a — d. 



1909. Monrxjraptus atavus, Jones, Quart. Jourii. Greol. Soc, vol. Ivi, p. 531, figs. 18 a, — d. 



Polypary very long and slender, with slight dorsal curvature proximally, but 



sometimes slightly ventral in its distal portion, widening gradually from an 



almost thread-like proximal extremity to a maximum breadth of 1*2 mm. 



Theca3 nine to eight in 10 mm., simple tubes with varying amount of 



sigmoid curvature and slight introversion of the apertural margin, which, 



however, diminishes distally; overlap never exceeding one-half the thecal 



length. 



Description. — Monog. atarns must have attained a very great length, probably 



fully 30 cm. or more. The widening is almost imperceptible in the proximal 



portion, but the greater number of distal fragments have a uniform breadth, so 



that the maximum width must have been attained at a comparatively early stage. 



The sicula is but rarely seen ; it seems to have a length of nearly 2 mm. and 

 to form the proximal termination of the polypary ; it does not reach to the level 

 of the aperture of th. 1. 



The thecas are relatively rather short (1*5— 2*5 mm. in length), and are of the 

 general type of those of the Leptograptida3 ; the sigmoid curvature of the walls 

 is constant, but appears to vary somewhat in amount in different specimens 

 (compare Figs. 270 e and 270 e). The distinct introversion of the apertural 

 margin of the proximal thecae disappears distally, when the thecal margins may 

 be even slightly everted. The number of tliec^ per centimetre is fairly constant 

 throughout the length of an individual, the overlap increasing with the general 

 increase in lensi'th of the tlieca. 



