Impressions of Carboniferous and Jurassic Nautiloids. 221 



the corresponding impression on the other side*. Botlx in 

 recent and fossil species the upper or anterior boundary of the 

 impression (a) is much more distinctly marked than the lower 

 or posterior \p), and accordingly the latter, as would naturally 

 be expected, is rarely preserved in fossil forms. On the inner 

 surface of the shell the anterior boundary of the impression is 

 marked by a very fine sharp ridge, which therefore in the cast 

 appears as a distinct groove. The impression of the annulus 

 is not quite so distinctly marked. 



(a) Carboniferous Species. 



Solenocheilus latiseptatus, de Koninck, sp.f 



Fig. 2 represents a natural cast (i nat. size) of Soleno- 

 cheilus latiseptatus from the Cement-stone of Carboniferous 

 age of the Arden Quarries, Nitshill, near Glasgow (B.M. 

 No. C. 2549 b). Only the anterior boundary (a) of the raus- 



Ficr. 2. 



cular impression is preserved, and that in the shape of a 

 groove ; but it shows that in this highly inflated form, with 

 an evenly rounded periphery, the annulus connecting the two 

 muscles of attachment was very short on the ventral side, so 

 that the muscles were rather more ventral than lateral ; 

 whereas in those species having a more or less flattened peri- 

 phery, e. g. N. pompiliusj N. polygonalis, &c, the reverse is 

 the case, the annulus being much longer on the ventral side. 



* The appearance of the shell-muscle and annulus as seen on the body 

 of the Nautilus, with which the impression here figured closely agrees, is 

 admirably figured in Sir Richard Owen's ' Memoir on the Pearly Nauti- 

 lus,' 1832, plate 1. 



t ' Faune du Calcaire Carbonifere de la Belgique ' (Ann. du Mus. 

 Roy. d'Hist. Nat. de Belgique, torn, ii.), 1878, p. 110, pi. xxii. figs. 1, 2, 3. 



