72 NATURAL HISTORY. (Fossils.) [NORTH 
moreover, present very remarkable forms. It is here that the visitor 
will find the nearest representatives of the teeth and spines arranged 
in the remaining Table Cases of fossil specimens. 
The second order, Ganoids, has derived its name from the lustre of 
the scales by which the fishes are protected, and is further distinguished 
by the nature of the substances of which the scales are composed, each 
scale consisting of a bony layer next the skin, and an enamelled ex¬ 
ternal portion. The most common form of scale here is rhomboidal. 
Ganoid fishes range from some of the oldest fossiliferous rocks up¬ 
wards ; are most abundant in the lowest jurassic formations and the 
Trias, and but feebly represented at the present time. In a large 
portion of the species of this division the vertebral,column passes into 
the upper lobe of the tail; it is bent upwards at a slight angle, and has 
the fin, or ray portion, springing entirely from the under side: this 
form of tail is termed Heterocercal, Others have the tail of the or¬ 
dinary structure ( Homocercal ), where the rays which support the fin 
spring from the terminal part of the vertebral column, and diverge 
vertically from that centre. In the Heterocercal fishes, the two lobes 
of the tail (the upper one formed, as it it were, by a portion of the 
body of the fish, and the lower consisting of a more or less pointed 
fin) are unequal; whilst in the Homocercal the lobes of the tail are 
generally equal. 
The order of Ganoids is commenced in the Wall Cases 1 and 2 
by the Cephalaspides, a family peculiar to the Devonian period. One 
of the most perfectly-preserved specimens of this family is the Cepha- 
laspis Lyellii, so named after its discoverer, by whom the specimen in 
Case I was presented. It is from the old red sandstone of Scot¬ 
land, from which formation and locality the remaining specimens of 
the family here exhibited have been derived. In the same Case are 
numerous specimens of Pterichthys; they are more or less crushed 
and imperfect, but in the examination of the entire series it will be 
seen that they were fishes in which the body was protected by large 
bony plates, was convex above, and flat beneath ; that the pectoral 
fins were represented by large bony appendages, situated close to the 
head; and that the tail was tolerably long and tapering, and furnished 
only with very small scales: traces of fins have been observed on this 
part. For a detailed account of the structure of these fishes the visitor 
may be referred to “ The Old Red Sandstone,” by Mr. Hugh Miller, 
and to a paper by Sir Philip Grey Egerton, in the Quarterly Journal 
of the Geological Society, Vol. IV. p. 302. The genus Coccosteus, 
of which there are several examples in Case 2, contains nearly-allied 
fishes. 
The second division of the Ganoid fishes—the Ccelacanthi, is illus¬ 
trated by numerous remarkable species contained in Cases 3 to 6. 
The name was suggested by the circumstance that the spines, or 
rather the rays of the fins, are hollow. The fishes of this family are 
found in the Devonian rocks, and in succeeding strata, up to the chalk. 
From the chalk of Sussex is a fine series of specimens, originally 
forming part of the Mantellian collection. They form the genus 
Macropoma. Among the species from the older formations attention 
may be particularly directed to the remains of a gigantic fish (Astero- 
levis Asmusii) found in the old red sandstone of Riga, and presented 
