Ca XXV.] 



MOUNTAIN LIMESTONE. 



407 



This information, derived from the color of the shells, is the more 

 welcome, hecause the Radiata, Articulata, and Mollusca of the Carbo- 

 niferous period belong almost entirely to genera no longer found in the 

 living creation, and respecting the habits of which we can only hazard 

 conjectures. 



Some few of the carboniferous mollusca, such as Avicula, Nucula, 

 Soiemya, and Lithodomus, belong no doubt to existing genera ; but the 

 majority, though often referred to living types, such as Isocardia, Turri- 

 tella, and Buccinum, belong really to forms which appear to have become 

 extinct at the close of the paleozoic epoch. Euomphalus is a character- 

 istic univalve shell of this period. In the interior it is often divided into 

 chambers (fig. 527 d), the septa or partitions not being perforated as in 



Fig. 527. 



Fig. 528. 



Euomphalus pentagulatus, Sowerby. Mountain Limestone. 

 a, Upper side ; b, lower, or umbilical side ; c, view showing mouth, which 

 Is less pentagonal in older individuals ; d, view of polished section, showing 

 internal chambers. 



foraminiferous shells, or in those having siphuncles, like the Nautilus. 

 The animal appears to have retreated at different periods of its growth 

 from the internal cavity previously formed, and to have closed all 

 communication with it by a septum. The number 

 of chambers is irregular, and they are generally 

 wanting in the innermost whorl. The animal of 

 the recent Turritella communis partitions off in 

 like manner as it advances in age a part of its 

 spire, forming a shelly septum. 



Nearly 20 species of the genus Bellerophon 

 (see fig. 528), a shell without chambers like the 

 living Argonaut, occur in the Mountain Lime- 

 Btone. The genus is not met with in strata of 



Bellerophon costatus, Sow. 

 Mountain Limestono. 



