108 
[Assembly 
tiary formationsj but the Transition affords us the converse of this pro- 
position; greater uniformity of temperature, a more perfect identity of 
organic remains, and rock masses of more uniform character. That the 
sea of one of these ancient eras was shallow, is proved not only by its 
universality, but by the ripple marks and fucoides which every where 
pervade the strata. A negative evidence is also supplied in the nature 
of the fossils. Dr. Buckland has shewn how admirably the complicated 
chambers, and the exterior surface, of the Ammonites were constructed, 
to resist the pressure of deep water. But shells of this genus were not 
created until more profound seas had resulted from changes in the confi- 
guration of the earth's surface, subsequent to the deposition of the coal 
formations. The Goniatite which has a plain exterior and septa sim- 
ply angulated, is unknown in that part of the Transition we have 
termed the Trenton group; and in the upper part of the series it is 
among the rarest of the univalves, abounding only in the Carboniferous 
epoch. The Cephalapods, which flourished in the lower transition, were 
reduced to the simpler structure of the genera Cyrtoceras^ Grthoceras, and 
a few kindred forms with plain arched septa like the Nautilus. There 
is also reason to believe that the Brachiopods of the Transition so diffe- 
rent from the prevailing form of the superior strata which constitutes 
the genus Terebratula, were, unlike the latter, denizens of very shoal wa- 
• ter. Immense numbers lived and were entombed with the remains of 
terrestrial plants on the margins of islands in the Carboniferous epoch. 
The bituminous shale of the coal measures is in some places stored with 
a species of Producta with long filiform spines, all the specimens having 
the two valves in their natural position, and the spines broken off from 
the shells simply by pressure of the superincumbent strata. They were 
never subjected to the action of a stormy surf, and yet land plants grew 
in the vicinity of the living shells; hence we infer a quiescent state of 
the waters around the islands of the transition eras. But one exception 
has been noticed, which occurs at Rochester, in a thin layer of lime- 
stone full of broken Pentamera; in other places single valves of bivalves 
occur, but the entire shells are so remarkably abundant, that whatever 
may have been the force of currents originating the breccias and con- 
glomerates, the waters were remarkably quiescent during the deposition 
of shales and limestones. 
As the student of geology has generally some difl^iculty in understand- 
ing the fossil Brachiopodous genera, it may be well to observe that we 
shall in our reports adopt the following names and divisions: 
Producta^ has one valve convex, the other concave, no cardinal area 
nor perforation under the beak. 
