HISTORICAL PALEONTOLOGY. 



Scallops (Aviculopecten and other allied forms), together with 

 numerous other types some of ancient origin, others repre- 

 sented here for the first time. Amongst the Gasteropods, we 

 find the characteristically Palaeozoic genera Macrocheilus and 

 Loxonema, the almost exclusively Palaeozoic Euomphalus, and 

 the persistent genus Pleurotomaria; whilst the free-swimming 

 Univalves (Heteropoda) are represented by Bellerophon and Por- 

 cellia, and the Pteropoda by the old genus Conularia. With regard 

 to the Carboniferous Univalves, it is also of interest to note here 

 the first appearance of true air-breathing or terrestrial Molluscs, 

 as discovered by Dawson and Bradley in the Coal-measures of 

 Nova Scotia and Illinois. Some of these (Conulus priscus) are 

 true Land-snails, resembling the existing Zonites; whilst others 

 (Pupa vetusta, fig. 128) appear to be generically inseparable 

 from the " Chrysalis-shells " 

 (Pupa) of the present day. 

 All the known forms three 

 in number are of small size, 

 and appear to have been local 

 in their distribution or in their 

 preservation. More import- 

 ant, however, than any of the 

 preceding, are the Cephalo- 

 poda, represented, as before, 

 exclusively by the chambered 

 shells of the Tetrabranchiates. 

 The older and simpler type of 

 these, with simple plain septa, 

 and mostly a central siphuncle, 

 is represented by the straight 

 conical shells of the ancient 

 genus Orthoceras, and the bow- 

 shaped shells of the equally 

 ancient Cyrtoceras some of 

 the former attaining a great size. 

 The spirally-curved discoidal 



shells of the persistent genus Nautilus are also not unknown, 

 and some of these likewise exhibit very considerable dimen- 

 sions. Lastly, the more complex family of the Ammonitida, 

 with lobed or angulated septa, and a dorsally-placed siphuncle 

 (situated on the convex side of the curved shells), now for the 

 first time commences to acquire a considerable prominence. 

 The principal representative of this group is the genus Gonia- 



Flg. 128. Pupa (Dendropupa) vetusta, 

 a Carboniferous Land-snail from theCoal- 

 measures of Nova Scotia, a, The shell, of 

 the natural size ; 6, The same, magnified; 

 c, Apex of the shell, enlarged ; d, Portion 

 of the surface, enlarged. (Alter Dawson.) 



