Ch. XXV.] FOSSILS OF THE MOUNTAIN LIMESTONE. 



519 



tins opinion lie is led by observing that in the existing seas the testa- 

 cea which have colors and well-defined patterns rarely inhabit greater 

 depths than 50 fathoms ; and the greater number are found where 

 {here is most light in very shallow water, not more than two fathoms 

 deep. There are even examples in the British seas of testacea which 

 are always white or colorless when taken from below 100 fathoms ; 

 and yet individuals of the same species, if taken from shallower zones, 

 are vividly striped or banded. 



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

 welcome, because the Radiata, Articulata, and Mollusca of the Car- 

 boniferous 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, JVucula, 

 Solemya, and Lithodomus, belong no doubt to existing genera ; but 

 the majority, though often referred to living types, such as Isocardia, 

 Turritella, and Buccinum, belong really to forms which appear to 

 have become extinct at the close of the Palaeozoic epoch. Euom- 

 phalus is a characteristic univalve shell of this period. In the inte- 



Fig. 576. 



Euomphalus pentagulatus, Sowerby. Mountain Limestone. 

 a. Upper side. &. Lower, or umbilical side. c. View showing mouth, which is less 

 pentagonal in older individuals, d. View of polished section, showing internal 

 chambers. 



rior it is often divided into chambers (fig. 576 d), the septa or parti- 

 tions not being perforated as in 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 



