Among the spiral univalve shells the extinct genus Euomphalus (see [fig. 391.]) is one of the commonest fossils of the Mountain limestone. In the interior it is often divided into chambers (see [fig. 391. d]); the septa or partitions not being perforated, as in foraminiferous shells, or in those having siphuncles, like the Nautilus. The animal appears, like the recent Bulimus decollatus, 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.
Fig. 391.
Euomphalus pentagulatus, Min. Con. 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.
Fig. 392.
Portion of Orthoceras laterale, Phillips. Mountain limestone.