266 BRITISH GRAPTOLITES. 



Diplograptus (Mesograptus) magnus, H. Lapworth. Plate XXXI, figs. 14 a — c. 



L900. Diplograptus magnus, H. Lapworth, Quart. Jouru. Geol. Soc, vol. lvi, p. 132, fig. 21. 



Polypary thick and robust, 4 cm. or more in length, narrow proximally, but 



widening for half its length until a maximum breadth of 4 mm. is attained, 



thence diminishing somewhat distally. Sicula conspicuous ; septum nearly 



complete. Thecse closely set, fourteen to twelve in 10 mm., showing a 



progressive change of shape in different parts of the polypary, overlapping 



one half to two thirds their extent; apertural margins undulate, narrow, 



concave, and everted in bi-profile view. 



Description. — The polypary is very thick, this being especially noticeable in 



specimens preserved in relief; it may attain a length of 4 cm or more, but shorter 



„ 100 , . ,, , specimens are of frequent occurrence ; it widens 



.b igs. \tS6 a and 6. — Mesograplus magnus, * 1 



fairly quickly till the maximum breadth is attained. 

 The sicula has a length of 2 mm., and in the 

 obverse aspect th. I 1 is seen to originate from a 

 point a little distance above the aperture, it grows 

 at first downward, then bends round abruptly to 

 grow outward and upward, while th. I 2 develops 

 from it, and, growing obliquely across the back of 

 the sicula, leaves it free for a considerable fraction 



a. Proximal end, obverse aspect, show- „ . , . , . , . , .... 



ing sicula and origin of th. 1 1 . ot its length on the right side. Ihe septum IS 



Ddol Farm, Rhayader : Gwastaden ■, . .-, n • -i i i i •, 



Series (Ddol shales). Coll. h. complete on the obverse side, but only makes its 



b. vuauLait, right half in relief, left appearance at the reverse side between the seventh 



half an impression. Ibid. thecal ^^ 



All the thecse in the initial region have very pronounced sigmoid curvature of 

 their walls in bi-profile view, but the free outer part is inclined ; the general 

 inclination of the theca is about 35° with respect to the axis of the polypary, the 

 overlap is one half, and the apertural margin appears slightly everted. In the 

 distal region the curvature becomes more uniform, the length of the theca? 

 increases up to 3 mm., the overlap to two thirds their length, and the apertural 

 margins appear still more decidedly everted. 



When compressed, the thecge of the proximal region show some points of 

 resemblance to those of a Glimacograptus, while the distal thecas suggest those of a 

 Petalogr aphis. 



Affinities. — Distal fragments of M. magnus thus resemble similar portions of 

 many Petalograjjti, but the polypary is not tabular, the thecse are more closely set, 

 and in the case of more or less perfect specimens the character of the proximal 

 thecse serves as a further distinguishing feature. 



