422 



BRITISH GRAPTOLITES. 



Monograptus Marri, Perner. Plate XLII, figs. 4 a—d. 



1897. Monograptus Marri, Perner, G-rapt. de Boiieme, pt. iiia, p. 21, pi. ii, figs. 5, 6, 10, 11, text-figs. 

 23—25. 



Figs. 284 a and b. — Monograptus Marri, 

 Perner. 



Polypary narrow, not exceeding 10 cm. in length, more or less straight and 



rigid, widening rapidly from an initial breadth of *5 mm. to a maximum of 



1'8 mm. which is then maintained. Sicula conspicuous. Thecge ten in 



10 mm., of the general type of M. priodon throughout, but with rather less of 



the theca involved in the hook, which occupies slightly more than half the 



breadth of the polypary. 



Description. — The polypary, while as a general rule stiff in appearance, shows 



sometimes a tendency to flexure which may occasionally be considerable. British 



examples seem to exceed the typical Bohemian form in length, though among them 



the maximum breadth of 1*8 mm. is but rarely 

 attained. Individuals with an average breadth of 

 about 1*5 mm. are of far commoner occurrence. 

 The normal breadth is attained fairly rapidly in the 

 proximal portion of the polypary, so that the width 

 is approximately uniform for the greater part of its 

 extent. 



The sicula has a length of 1*2 mm. reaching just 

 half way between th. 1 and th. 2. 



The thecaa number ten in 10 mm. throughout, 

 from the proximal to the distal extremity of the 

 polypary. Nearly two-thirds of the total thecal 

 length is involved in the hook, which occupies rather 

 more than one-half of the width of polypary. The 

 interthecal line between any two thecae is short, and 

 is as a rule quite inconspicuous, in marked contrast 

 to what obtains in M. priodon. As in M. priodon, 

 there is a considerable amount of torsion of the thecal axis, so that in one aspect 

 (the reverse) there is often a deceptive appearance of tapering of the theca 

 towards its apertural margin. Sometimes, too, the effect of compression on the 

 twisted theca makes it appear as if the thecal axis had curved through a greater 

 angle than 180°. Not only so, but in some examples the thecal angle appears to be 

 slightly under or slightly over this, and it can, therefore, hardly be doubted that 

 this effect is one due merely to compression and is a deceptive appearance. 



Affinities. — Monog. Marri is closely related to both ill. priodon and to M. pandus. 

 It differs from M. priodon in the dimensions of the polypary as a whole, being both 

 shorter and narrower ; as regards the thecas, though of the same general type as 





% 



a. Proximal end in low relief, showing' 



sicula. Enlargement of part of 

 PI. XLII, %. 4a. 



b. Distal tliecee in partial and low 



relief. Enlargement of part of PI. 

 XLII, fig. 4 c. 



