378 
ELEMENTS OF GEOLOGY. 
The first column marks the last appearance of several gen¬ 
era which are characteristic of Palaeozoic strata. The second 
shows those genera which are characteristic of the Upper 
Trias, either as peculiar to it, or, as in the three cases mark¬ 
ed by asterisks, reaching their maximum of development at 
this era. The third column marks the first appearance in 
Triassic rocks of genera destined to become more abundant 
in later ages. 
It is only, however, when we contemplate the number of 
species by which each of the above-mentioned genera.are 
represented that we comprehend the peculiarities of what is 
commonly called the St. Cassian fauna. Thus, for example, 
the Ammonite, which is not common to older rocks, is repre¬ 
sented by no less than seventy-three species; whereas Loxo- 
nema, which is only known as common to older rocks, fur¬ 
nishes fifteen Triassic species. Cerithium, so abundant in ter¬ 
tiary strata, and which still lives, is represented by no less 
than fourteen species. As the Orthoceras had never been 
met with in the marine Muschelkalk, much surprise was nat¬ 
urally felt that seven or eight species of the genus should 
appear in the Hallstadt beds, assuming these last to belong 
to the Upper Trias. Among these species are some of large 
dimensions, associated with large Ammonites with foliated 
lobes, a form never seen before so low in the series, while 
the Orthoceras had never been seen so high. 
On the whole, the rich marine fauna of Hallstadt and St. 
Cassian, now generally assigned to the lowest members of 
the Upper Trias or Keuper, leads us to suspect that when 
the strata of the Triassic age are better known, especially 
those belonging to the period of the Bunter sandstone, the 
break between the PalaBozoic and Mesozoic Periods may be 
almost effaced. Indeed some geologists are not yet satisfied 
that the true position of the St. Cassian beds (containing so 
great an admixture of types, having at once both Mesozoic 
and Palaeozoic affinities) is made out, and doubt whether 
they have yet been clearly proved to be newer than the 
Muschelkalk. 
Muschelkalk.—The next member of the Trias in Germany, 
the Muschelkalk^ which underlies the Keuper before described, 
consists chiefly of a compact grayish limestone, but includes 
beds of dolomite in many places, together with gypsum and 
rock-salt. This limestone, a formation wholly unrepresent¬ 
ed in England, abounds in fossil shells, as the name imj^lies. 
Among the Cephalopoda there are no belemnites, and no am¬ 
monites with foliated sutures, as in the Lias, and Oolite, and 
the Hallstadt beds; but we find instead a genus allied to 
