72 PALEONTOLOGY 



found in the newer tertiary of Italy, with Vaginella (fig. 19, 12), 

 a form allied to Cuvieria. But the occurrence of Ptcropoda in 

 the older rocks is attended with considerable obscurity. Eccu- 

 liomphalus is like an incompletely convoluted EuomjjJialus ; 

 Maclurca is like Euomplialus with a depressed spire ; the shells 

 called Theca are slender and conical ; Pterothcca has a wing- 

 like expansion ; and Conularia (fig. 17, 10) is a four-sided 

 sheath, with the apex partitioned off, as in the recent Cuvieria. 

 If really pteropodous, these shells are the giants of the order. 



Nucleobranchiata.- — Those fossil univalves, which in their 

 symmetry resemble the Nautilus, but are unfurnished with 

 air-chambers, have been compared to the recent Heteropoda 

 (or Nucleobranchiata, Bl.), and especially to that division 

 typified by the tiny Atlanta, in which the animal can with- 

 draw itself completely into its shell, and close the aperture 

 with an operculum. The genus Porcellia, characteristic of the 

 carboniferous age, has a discoidal shell, with a spiral nucleus 

 projecting, as in Atlanta, from the right side ; the whirls are 

 exposed, and marked with a narrow band along the back, 

 ending in a deep slit (fig. 17, 6). Another genus (BcUerojJion) 

 resembles the recent Oxygyrus in its more globose form, with 

 a similar narrow umbilicus on either side (fig. 17, 7) ; some- 

 times the shell is thin and the aperture expanded, like a 

 trumpet, whilst other species are globular and solid ; the for- 

 mer may have been tenanted by large animals living at the 

 surface of the open sea, the latter seem to have been more 

 adapted to protect their owners crawling over the bottom, for 

 it can scarcely be insisted that all were necessarily floaters on 

 account of their organization. The species of Bcllcvojdion are 

 numerous in all the palaeozoic rocks, and some of the smaller 

 kinds appear to have been gregarious : those with disconnected 

 whirls have been called Cyrtolitcs (Conrad.) The Bdlerqphma 

 of d'Orbigny (fig. 18, n), is a minute shell found in the gault, 

 The other division (Firolida^ consists of Mollusks in which 



