530 Dr. H. P. Blackmore— 



In the neighbourhood of Salisbury, the zone of BelemniteUa 

 qiiadrata is largely developed, as well as the smaller, more limited 

 bed which contains BelemniteUa lanceolata : a carefully-collected 



series of fossils from these two zones soon brought out. the fact that 

 Aptychus leptophyllus and BelemniteUa lanceolata always occurred 

 together, and that Aptijclnis Portloclcii and BelemniteUa quadrata were 

 similarly associated. A further study of the specimens themselves 

 revealed what this probably meant, viz., that these two Aptychi were 

 the forward extension of the phragmocone, called by Huxley the 

 pro-ostracum, of BelemniteUa lanceolata and BelemniteUa quadrata 

 respectively. Let us now examine more closely and see whether 

 the facts bear out this interpretation, and, for the sake of convenience, 

 begin with Aptychus leptophyllus. as related to BelemniteUa lanceolata. 



If a specimen of the latter, sufficiently well preserved to show the 

 form of the alveolar opening of the guard, be carefully examined, 

 it will be noticed that the lateral curve of the anterior edge exactly 

 fits the posterior end of Aptyclms leptophyllus ; and further, that these 

 corresponding edges are similarly ended by roughened margins 

 (Figs. 1 and 2) ; also, on the inner surface of the outer margin of the 

 proximal end is a well-marked curved line (more apparent in some 

 specimens than others), evidently caused by a fold of membrane 

 which attached this part of the shell to the latero-dorsal end of the 

 alveolus. The semi-crystalline structure and mineral condition of 

 Aptyclms leptophyllus are identical with the alveolar lining of 

 B. lanceolata, both differing from the structure of the guard, 

 corresponding in this respect with the analogous structure of the 

 conotheca of Belemnites paxillosus, B. inornatns, and B. ellipticus, 

 from the Lias and Oolite ; furthermore, the singular and characteristic 

 curves on the surface of the conotheca of the first of these species 

 closely resemble the well-marked curved lines of growth seen on 

 the inner concave surface of Aptychus leptophyllus. The outer surface 

 of the proximal end of most of the aptychi shows distinct marks of 

 lamination, indicating the secretion of successive layers of shell, 

 deposited by the membrane as the animal increased in age. 



This is an important point to notice, as it clearly proves that the 

 shell grew from this end of the apt3'chus, and not from the straight 

 lateral edge, as would have been the case if these aptychi represented 

 the opercula of Ammonites. The well-marked curved lines of growth 

 on the inner concave surface, as well as the direction of the few 

 radiating longitudinal lines, also prove the same point. 



The outer surface of the aptychi is smooth, convex, and thickened 

 towards the outer edge, which in a side view presents a double 

 curve; the distal end is terminated by a thickened, rounded, smooth 

 ridge. The straight inner side is very thin, sharply curved inwards 

 at its margin ; some specimens from the Salisbury chalk-pits show 

 the two halves united along this margin, one lapping over the other, 

 the junction forming a straight groove which was probably a con- 

 tinuation of the ventral sulcus or the siphuncle. 



The alveolar end of the guard of B. lanceolata is general!}' much 

 paler in colour than the rest of the guard ; but at about a quarter of 





