i8 FIELD MUSEUM OF NATURAL HISTORY 
Trcmatopora 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, Stropho stylus, etc., among 
Gastropods are typical molluscs of the period. 
Many specimens are shown illustrating the size and form of 
Silurian Cephalopods. The several sections show that the 
shell was divided by cross partitions into chambers. The ani- 
mal occupied only the end chamber, but a long tube or siphuncle 
connected the others to its body. The straight forms, such as 
Orthoceras, predominate but the curved Cyrtoceras and Onco- 
ceras, the open coiled Gyroceras and the closely coiled Nautilus 
and Trochoceras occur and specimens are shown. Phragmoceras 
and Gomphoceras, having a constricted aperture, constitute a new 
type of Cephalopod. 
Silurian Trilobites are fewer in number and somewhat small- 
er 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 
fishes of the present day, however. Part of the Ganoids were 
covered with bony scales, had teeth of reptilian character, and 
jointed, paired fins. Others, the Placoderms, were protected 
by bony plates covering the head and fore part of the body. 
The Placoids had cartilaginous skeletons, no scales, no gill 
covers, and many of their characters were embryonic. In most 
cases only teeth and spines are found fossil. Mesacanthus and 
