THE TRIASSIC. PERIOD. 213 
tremely complex; whilst in the Ceratite there is an inter- 
mediate state of things, the special feature of which is, that 
those foldings which are turned towards the mouth of the 
shell are merely rounded, whereas those which are turned 
away from the mouth are characteristically toothed. The 
genus Ceratites, though principally ‘Triassic, has recently been 
recognised in strata of Carboniferous age in India. 
From the foregoing it will be gathered that one of the most 
important points in connection with the Triassic AZollusca is 
the remarkable intermixture of Palzeozoic and Mesozoic types 
which they exhibit. It is to be remembered, also, that this 
intermixture has hitherto been recognised, not in the Middle 
Triassic limestones of the Muschelkalk, in which —as the 
oldest Triassic beds with marine fossils—we should naturally 
expect to find it, but in the St Cassian beds, the age of which 
is considerably later than that of the Muschelkalk. The 
intermingling of old and new types of Shell-fish in the Upper 
Tnias is well brought out in the annexed table, given by Sir 
Charles Lyell in his ‘Student’s Elements of Geology’ (some 
of the less important forms in the table being omitted here) :— 
GENERA OF FossIL MOLLUSCA IN THE ST CASSIAN 
AND HALLSTADT BEDS. 
Characteristic of Triassic 
~ Common to Older Rocks. Common to Newer Rocks. 
Rocks. 
Orthoceras. Ceratites. Ammonites. 
Bactrites. Cochloceras. Chemnitzia. 
Macrocheilus. Rhabdoceras. Cerithium. 
Loxonema. Aulacoceras. Monodonta. 
Holopella. Naticella. Spheera. 
Murchisonia. Platystoma. Cardita. 
Porcellia. Halobia. Myoconcha. 
Athyris. Ho6rnesia. Hinnites. 
Retzia. ‘ Koninckia. Monotis. 
Cyrtina. Scoliostoma. Plicatula. 
Euomphalus. Myophoria. Pachyrisma. 
(The last two are princi- | - Thecidium. 
pally but not exclus- | 
ively Triassic. ) 
Thus, to emphasise the more important points alone, the Trias 
has yielded, amongst the Gasteropods, the characteristically 
Paleozoic Loxonema, Holopella, Murchisonia, Euomphalus, and 
Porcellia, along with typically Triassic forms like //atystoma 
and Scofostoma, and the great modern groups Chemmnifzia and 
Cerithium. Amongst the Bivalves we find the Paleozoic 
Megalodon side by side with the Triassic Halobia and AZyo- 
phoria, these being associated with the Cardite, Hinnites, 
Plicatule, and Trigonie of later deposits, The Brachiopods 
