JOHN HOPKINSON ON D1PLOGRAPSUS PRISTIS. 



205 



Fig. 1. 



there being ten to the inch. They appear to have budded from the 

 periderm at right angles to the hydrothecas, and thus have caused 

 the polypary to be unevenly compressed. The most perfect are 

 pear-shaped in form, l-6th of an inch long, and at their narrow 

 end, by which they are attached, about l-30th of an inch wide. 

 They have apparently been bounded by a single marginal fibre, 

 which is slightly thickened at its edges, and, where the pyrites is 

 removed, has impressed a fine 

 double groove on the surface of 

 the shale. If the fibres were 

 slender tubes this appearance 

 would naturally be presented ; for 

 their outer margins would offer 

 the greatest resistance to com- 

 pression. The so-called solid axis 

 of the graptolite frequently pre- 

 sents a similar appearance. At 

 the proximal end of the polypary 

 these fibres only are preserved, 

 the oldest, or first-formed gono- 

 thecae having fulfilled their func- 

 tion and perished. The distal ex- 

 tremity of even the most perfect 

 is not clearly defined, the impres- 

 sion of the capsule in most cases 

 becoming gradually less percepti- 

 ble from the proximal to the distal 

 end. Sometimes the capsules are 

 irregularly ruptured, their torn 

 jagged edges being distinctly 

 seen, while one has split along its marginal limit, along the line of 

 the marginal fibre, which appears to have parted abruptly near the 

 distal end of the capsule at one side, and split acutely for some dis- 

 tance along the other side. This would appear to indicate that the 

 capsule may be composed of two membranes joined together at their 

 edges, through which the fibre, if it be not merely a tube formed 

 by a kind of double marginal seam, has run. In no case can a 

 distinct unruptured distal orifice be traced. 



The gonotheca3 present other peculiar appearances. Towards 

 their proximal end they are sometimes longitudinally corrugated 



Journ. Q M. C. No. 15. p 



Diplograpsus pristis 

 with reproductive capsules. 

 Magn. 3 diameters. 



