252 



Br. G. Holm — On BaUoceras. 



there is a thin opaque layer of a light greyish-yellow colour. The 

 septa, therefore, corresponding to the different layers, are white in 

 the figures, with black boundary-lines. The wall connecting the 



Undoceras BurcJianlii, Dew. The island of Oland, Grey Lituites Limestone. 



Fig. 1. — Tangential section through the middle of the siphon. The plane of the 

 section is not exactly at right angles to the middle plane of the conch, nor is it 

 exactly parallel to the longitudinal axis of the siphon. The latter circumstance 

 is the reason why the siphon appears to taper towards the distal end. Drawing 

 after a photograph. Enlarged three diameters. 



Fig. 2. — Section in the middle plane. Enlarged two diameters. 



septal tubes (funnels) is of about the same colour and appearance 

 as the outer laj'ers of the septa (like them black in the figures). 

 The loculi are filled with coarse-grained calc-spar ; the siphon with 

 the rocky substance. The septal tubes (funnels) are short, and 

 extend only the fifth of the distance to the next septum, ending in 

 a sharp edge. The intervening connective wall is thicker than the 

 septa, opaque, sharply defined inwards, while the outside, even 

 where the specimens are not weathered, is decomposed, having no 

 even or clear-cut surface. At the posterior part it attaches itself 

 to the inner side of the posterior septal tube (funnel) ; in the 

 anterior part it encloses the outer side of the anterior septal tube 

 (funnel). The strictures and the peculiar appearance of loose 

 siphons is thus easily explained, as the siphon between the septal 

 tubes (funnels) is easily removed from the enclosing stone and from 

 the short septal tubes (funnels). The strictures are therefore 

 impressions from the septal tubes (funnels). 



In spite of its apparent resemblance to Endoceras — through the 

 comparatively large and marginal siphon (diam. f-f of that of 

 the conch) — it appears that the species above described, considering 

 the structure of the ectosiphon, does not even belong to the same 

 subdivision {Holochoanoida, Hyatt) of Nautiloidea as Endoceras, but 

 has its nearest relations in the family Orthoceratidfe of the sub- 

 division EllipocJioanoida, Hyatt. It follows that even within the 

 latter family there must occur species with a large marginal siphon, 

 although they have been overlooked on account of their apparent 



