16 FIELD MUSEUM OF NATURAL HISTORY 
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 
Ptyctodus are Placoids. Paleospondylus is considered an ancient 
lampry. Holoptychius and Palceoniscus give a good idea of 
the general form of the scaled Ganoids and Macropetalichthys, 
Pterichthys, Coccosteus are representative Placoderms. 
Aside from its fishes the Devonian period is noted for the 
appearance of the land plants, such as Neuvopteris and Alethop- 
teris (Ferns), Asterophyllites (Horsetails), Adiantites, etc. 
Dictyophyton is a reticulate Sponge. Both the Honeycomb 
corals, such as Favosites, Michelinia, Alveolites, etc., and the 
Cup corals Heliophyllum, Cyathophyllum^ Acervularia, Cysti- 
phyllum, Zaphrentis, Campophyllum, etc., were very abundant 
both as to number of species and individuals. An especially 
large number of species of Favosites, Heliophyllum and Zaphr- 
rentis is shown. 
Crinoids were comparatively rare in Devonian times. Cupres^ 
socrinus, Megistocrinus and Hypsocrinus represent the class in 
the collections. Roemeraster, Aspidostonia, Ophiura, etc., are 
Star-fishes. Brachiopods reached their greatest development in 
this period and all the typical forms are represented: perhaps 
the most noticeable of these is the winged Spirifer of which 
many species are shown. 
Pelecypods and Gastropods are abundant but exhibit no 
decided changes from the preceding period. 
Cephalopods are represented by Orthoceras, Gomphoceras, 
Gyroceras, etc., as before, but to these is added the Goniatites, 
which is a closely coiled form having the juncture of the septa 
and shell forming a zigzag instead of a straight suture. 
Mississippian or Subcarboniferous Period. — In the period in- 
tervening between the Devonian and the period of the deposition 
of coal (Pennsylvanian) occurred the so-called Subcarboniferous 
or Mississippian period. Especially characteristic forms of this 
period in some localities were the crinoids which were of various 
and striking forms. An exceptionally fine collection of these 
