DEPARTMENT OF GEOLOGY 
15 
rentis, Stret^telasma, Omphyma, Eridophyllum, etc., are also 
shown. 
Crinoids and Cystoids showed a wonderful development both 
as to size and number in this period. Eucalyptocrinus^ Perie- 
chocrinuSj Siphonocrinus, CallicrinuSj Caryocrinus, HolocystiteSj 
etc., are some of the prominent genera represented. Cladopora, 
Tremafopora and Fenestella illustrate Bryozoans. They were 
not abundant in this period. 
As examples of Brachiopods may be noted, besides the con- 
cavo-convex Stropheodonta and the globose forms Meristina, 
Pentamerus, Atrypa, Whitfieldella, etc., of the preceding period, 
the winged form Spirifer. This is abundant. 
Mytilarca and Megalomus among Pelecypods and Platy- 
ceras, Platyostoma, Tremanotus, Strophostylus^ etc. among Gas- 
tropods are typical molluscs of the period. 
Many specimens are shown illustrating the size and form of 
Silurian Cephalopods. Sections show that the shell was divided 
by cross partitions into chambers. The animal occupied only 
the end chamber, but a long tube or siphuncle connected the 
others to its body. The straight forms, such as Orthoceras, 
predominated but the curved Cyrtoceras and Oncoceras, the open 
coiled Gyroceras and the closely coiled Nautilus and Trochoce- 
ras occur and specimens are shown. Phragmoceras and Gom- 
phoceras, having a constricted aperture, constitute a new type of 
Cephalopod. 
Silurian Trilobites are fewer in number and somewhat smaller 
than those of the former period but show a marked tendency 
to ornamentation. Many of the old genera continue and to 
them are added Phacops, Encrinurus, Proetus, etc., which are 
represented by specimens. The Eurypterids form a group of 
crustaceans which first appear in this period. They resemble 
the scorpions in form. They are the ancestors of the modern 
horse-shoe crab. Eurypterus, Pterygotus, etc., belong to this 
group. They are represented in the collection by an especially 
fine series of specimens. 
Devonian Period or Age of Fishes. — The fishes which by their 
size and abundance characterized this age, belonged to two 
orders — Ganoids, represented at the present day by the gar- 
fish and sturgeon, and Placoids, the order which includes sharks, 
skates and rays. They differed in many respects from the 
