DIPLOGRAPTUS. 261 



Diplograptus (Mesograptus) multidens, sp. nov. Plate XXXI, figs. 9 a — d. 



Polypary 5 cm. or more in length, increasing rapidly in breadth till the 



maximum of 3*5 — 4 mm. is attained ; with flattened tube running through 



centre, and emerging at distal extremity. Sicula small ; pair of proximal 



thecse adorned with unsymmetrically disposed spines. Thecas closely 



set, eighteen to fourteen in 10 mm., being in all profile views of a general 



Climacograptiis type in the proximal region of the polypary, but acquiring 



the more typical Orthograptus characters towards its distal extremity 



with great overlap and undulating apertural margins. 



Description. — The polypary is often of considerable length, but is quite narrow 



in the proximal region, where it never exceeds 1*2 mm. in breadth; it widens 



rapidly, however, in the first 1*5 cm. of its growth to attain its maximum breadth, 



which is then usually maintained for the greater part of its extent, though there 



is a tendency in some specimens to narrow distally. 



The sicula lias a length of only 1 mm. Th. I 1 originates close to the aperture 

 and grows almost at once outward and upward ; it has an everted apertural edge 

 prolonged into a spine, and in this respect differs from all the other thecse of the 

 proximal region ; th. I 3 grows at first obliquely across the sicula and then directly 

 upward for a short distance, giving off a spine where upward growth commences. 



In profile views the remaining theca? in the proximal region all show the 

 characteristics of those of a Climacograptiis — viz., pronounced sigmoid curvature 



of the ventral margin, straight but inclined free 



Pig. 178. — Mesograptus multidens, nov. 



edge, and concave horizontal or slightly everted 



apertures. As the polypary grows, however, the 



«5V J s? sigmoid curvature decreases, the angle of inclination 



v\ sinks to 30°, and the apertures become more and 



• ! 



more everted, until a theca like that of Orthograptus 

 is produced; so that while young polyparies and 



proximal fragments closely resemble those of a 

 Proximal end, reverse aspect, in very Climacograptus (except for th. I 1 ), distal fragments 



low relief, showing- the appearances ■, n >i 1 j • j- p i i r\ n 1 



presented by the earlier thecee have all the characteristics of a typical Orthograptus. 



^IZ^^Tl&eC™ Xl1 tlie Proximal region the theca? are small and 



very closely set, but distally they have a length of 

 25 mm. and overlap two thirds to three fourths of their extent. The flattened 

 tube which runs throughout the polypary and emerges at the distal extremity, 

 is very conspicuous in this species. 



Affinities. — M. multidens somewhat resembles Glimacog. tubidiferus, on account 

 of the presence of this broad central tube, and has very frequently been confounded 

 with it. It should, however, be readily distinguished, not only by the progressive 



