532 Dr. H. P. Blackmore—Aptychi from the Upper Chalk. 



Mr. Sharpe described in the Memoir of the Palreontological Society 

 under the name of Aptychus Vortloclcii, differ from the pro-ostracum 

 of B. lanceolata. There is, however, to be noticed the same 

 appropriate correspondence in regard to size, between these small 

 aptychi and the small contracted alveolar opening of B. quadrata ; 

 the largest specimen measuring in length IS mm. and the smallest 

 only 7 mm. The straight inner edge is very different from that of 

 Apt. leptophyllus, being bent into an angular process which laps over 

 the similar projection of the other shell — generally the left laps 

 over the right — and when thus united they form on the inside 

 a groove which corresponds with the internal groove previously 

 described in the pro-ostracum of B. lanceolata. (See Figs. 10-12.) 



There is no evidence how these small pro-ostraca were attached ; 

 but probably the attachment was loosely by membrane, not so near 

 the guard as in B. lanceolata, and hence were even more easily 

 detached and separated from the guard after the death of the 

 Belemnite. 



No trace of mandibles has been hitherto found associated with 

 B. quadrata ; the beak was possibly horny and not of the same com- 

 position as the guard or pi'o-ostracum, hence it was not preserved, 

 and has left no trace behind. 



After having established the fact of the relationship of B. lanceolata 

 and B. quadrata to the two aptychi found with them, attention was 

 paid to the higher zone of the Chalk, and here it was discovered 

 that Aptychus rugosus was associated with B. mucronata. As the 

 guard and alveolar cavity of this species bear a close resemblance 

 to the same parts of B. lanceolata, so Aptychus rugosus in many 

 ways — in outline, general form, and relative size — resembles Apt. 

 leptophyllus. There is the same thin, straight inner edge curved 

 to lap over the margin of its fellow shell, and the curved lines on 

 the inner concave surface are nearly identical. Externally, as 

 the specific name indicates, the surface is marked with a seines of 

 roughened ridges, arranged in irregular parallel lines, which some- 

 what follow the outline of the upper and outer sides of the shell ; 

 these tuberculated ridges do not extend to the outside, which is 

 terminated by a thin, flat, bevelled edge. The peculiar sculpturing 

 of the external surface is probably due to the impression made by 

 the roughened membrane which originally covered this extension 

 of the phragmocone, and bears a close resemblance to the granular 

 surface, so often seen on the guard of B. quadrata, and also very 

 marked in the upper part of the guard of a large specimen of 

 B. mucronata, from Westphalia, in the Natural History Museum, 

 Cromwell Road, which is undoubtedly secreted by the membrane 

 covering it. 



Briefly, therefore, it appears that — 



(1) Aptychus rugosus is the pro-ostracum of Belemuitella mucronata. 



(2) Aptychus leptophyllus is the same part of Belemuitella lanceolata. 



(3) Aptychus Portlockii is the pro-ostracum of Belemuitella quadrata, 



(4) The large, coarsely punctate aptychus from the Marsupite 

 zone is the true Aptychus of Ammonites leptophyllus. 



