MOUNTAIN LIMESTONE.—MOLLUSCA. 
435 
which ornamented the living shell. These colored bands are 
also preserved in several lamellibranchiate bivalves, as in 
Aviculopecten (Fig. 485), in which dark stripes alternate 
with a light ground. In some also of the spiral univalves 
the pattern of the original painting is distinctly retained, as 
in Pleurotomaria (Fig. 486), which displays wavy blotches, 
resembling the coloring in many recent Trochidse. 
Some few of the carboniferous mollusca, such as Avicula^ 
Niiciila (sub-genus Gtenodonta)^ Solemyci^ and Lithodomus, 
belong no doubt to existing genera; but the majority, 
though often referred to as living types, such as Isocardia^ 
Turritella^ and Buccinum^ belong really to forms which ap¬ 
pear to have become extinct at the close of the PaleBozoic 
epoch. Euomphalus is a characteristic univalve shell of this 
Euomphaluspentangulatus, 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. 
period. In the interior it is divided into chambers (Fig. 487, 
f/), the septa or partitions not being perforated as in forami- 
niferous shells, or in those having siphuncles, like the Nauti¬ 
lus. The animal appears to have retreated at different peri¬ 
ods 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 ad¬ 
vances in age a part of its spire, forming a shelly septum. 
