520 



FOSSILS OF THE 



[Cn. XXV. 



Fig. 577. a S eptum. The number of chambers is irregu- 



lar, and they are generally wanting in the in- 

 nermost whorl. The animal of the recent Tur- 

 ritella communis partitions off in like manner as 

 it advances in age a part of its spire, forming a 

 shelly septum. 



Nearly twenty species of the genus Bellero- 

 Belierophon costatus, Sow. phon (see fig. 5.77), a shell without chambers 



Mountain Limestone. vi ' j.i " i • ■ a j. *j.itvt ± • 



like the living Argonaut, occur in the Mountain 

 Limestone. The genus is not met with in strata of later date. It is 

 most generally regarded as belonging to the Heteropoda, and allied to 

 the Glass-Shell, Carinaria ; but by some few it is thought to be a 

 simple form of Cephalopod. 



The carboniferous Cephalopoda do not depart so widely from the 

 living type (the Nautilus) as do the more ancient Silurian representa- 

 tives of the same order ; yet they offer some remarkable forms 

 scarcely known in strata newer than the coal. Among these is 

 Orthoceras, a siphuncled and chambered shell, like a Nautilus un- 

 coiled and straightened (fig. 578). Some species of this genus are 



Portion of Orthooeras laterals, Phillips. Mountain Limestone. 



several feet long. The Goniatite is another genus, nearly allied to the 

 Ammonite, from which it differs in having the lobes of the septa free 

 from lateral denticulations, or crenatures ; so that the outline of these 

 is continuous and uninterrupted. 



Fig. 579. 



Fig. 580. 



Goniatites crenistria, PhilL Mountain Lime- 

 stone. H". America ; Britain ; Germany, &c. 

 a. Lateral view. 

 o. Front view, showing the mouth. 



Goniatites evolutus, Phillips. 



Mountain Limestone. 



Yorkshire. 



The species represented in fig. 579 is found in almost all localities, 

 and presents the zigzag character of the septal lobes in perfection. 



In another species (fig. 580), the septa are but slightly waved, and 

 so approach nearer to the form of those of the Nautilus. The dorsal 



