io82 The American Naturalist. [December, 
of these groups, prefacing the examination by a reference to the 
mode and condition of preservation of one of them in particular, 
the corals. 
The exposure of Lower Silurian rocks at Cincinnati is noted 
for the excellent preservation of its fossils and for their great 
abundance. Especially is it noted for the great number of indi- 
viduals of brachiopods and corals. The rocks which now make 
up the formation were most likely deposited upon a ridge in the 
bottom of the ocean, previously formed by the contraction of the 
earth's crust. A warm current of water sweeping over this brought 
quantities of food, and enabled the animal forms to increase and 
multiply. Gradual growth, assisted, probably, by elevatory forces, 
must at times have brought the rocks up to or near the surface 
of the water, subsequent sinking allowing additional matter to be 
deposited. There seems little reason to doubt that these rocks 
were deposited in shallow water, and under conditions which 
brought them at times even above its surface. 
As already stated, the corals are among the most abundant 
forms. They occur at certain localities literally in thousands, 
and where the shale has crumbled through weathering, they cover 
the ground so one can gather them up by the handful. Attention 
was first directed to these corals by a scientific man about 1870, 
and since then they have been the special study of a number of 
palaeontologists.^ First one and then another undertook their 
study, until now a rather thorough knowledge of the group is 
the result. At first the few described species were referred to 
the genus Chaetetes. Later investigations caused them to be 
placed in the genus Monticulipora, and a special family was estab- 
lished for them, for which reason they have frequently been called 
the " Monticuliporoids." At first this single genus Monticuli- 
pora, like Chaetetes, was enough. Then it was divided into five 
sub-genera, this classification being based largely on internal 
structure. Next we find it divided into twenty or more distinct 
genera, a host of species described, and the whole removed from 
the corals to the polyzoa. Since the first sub-division the work 
:. O. Ulrich. 
ngs, especially, of U. P. Jai 
