26 
NORTH GALLERY. 
[upper 
also supposed to be of the New Red Sandstone formation, with equally 
remarkable impressions of various dimensions, called Ornithichnites, 
being regarded as the foot-marks of birds. They occur in the Sand- 
stone beds near Greenfield, Massachusetts, at Turner s Falls, in the 
Connecticut Eiver. 
ROOM II. 
The classification of the Fossil Fishes, arranged in this room, is 
chiefly in accordance with that proposed by M. Agassiz, in his great 
work, entitled ''PiecJierches sur les Poissons Fossiles,'" \yiih some mo- 
difications founded upon the later systems proposed by Professors Miiller 
and Owen. The series commences with the Placoid Fishes, or those of 
the Shark and Ray tribes, in which the skin is protected by rounded 
(often star-shaped) and very hard scales, having frequently a raised 
point, and sometimes a thorn-like prickle in the centre, as may be 
seen in the scales of the Thornback and some other fishes of the 
Skate tribe. The upper division of the tail is prolonged beyond the 
lower lobe, and is supported by a continuation of the vertebral 
column — a form of tail which is termed Hetero cereal, and which is 
most commonly found in all the orders of fishes of the middle and older 
Geological formations ; but which (if we except the Sharks and Rays) 
is rarely met with in the existing species of fishes, in which the " ho- 
mocercal " tail, or that with the two lobes equal, prevails. 
The skeleton of the Placoids being more or less gristly, and in the 
same degree perishable and incapable of fossilization, the remains of 
those fishes consist chiefly of the defensive spines, scales, and teeth ; 
these objects, being mostly of small size, will be found in the Cases 
under the windows, and in Case 7 at the end of the room. 
The Fishes called Ganoids have derived their name, and the 
character of their order, from the lustre of their very hard, enamelled 
scales ; and it is by these parts that they are chiefly represented in 
the fossil state. Tlie most common form of scale in this order is the 
rhomboidal, but the pattern of the external markings varies in almost 
every species. 
Ganoid Fishes range from the newest Silurian strata upwards ; are 
most abundant in the lower Oolitic formations, diminish in the cre- 
taceous beds, and arc reduced to very few genera existing at the present 
time. 
The order is commenced in Wall Case No. 1, by the Cei^halaspides, 
a family peculiar to the Devonian period. The species of which it is 
composed were fishes in which the body was protected by large bony 
plates, was convex above, and flat beneath ; the pectoral fins were 
represented by iai'ge bony appendages, situated close behind the head ; 
and the tail was tolerably long, tapering, and furnished with small 
scales. Tiie second family of the order, the CailacantJii, so called 
from the spines of tlieir fins being hollow, occupy the compartments 
.*] to 5 of the same Wall Case. In the Gth compartment are arranged 
the Dipterines, the third family, including fishes of the Old Red Sand- 
