31 
have had three species in the coal-measure rocks of Ohio, 
C. elegans, C. ornatus, and C. robustus. It is interesting and 
curious to note the very marked resemblance existing between 
this genus, and Undina in the Jurassic, and MMacropoma in 
the Cretaceous, amounting to something almost like generic 
identity, as pointed out lately by Prof. Huxley. The hollow 
fin-spines, whence the name of the type-genus was formed, 
the ornamented bony head-plates, the ossified air-bladder, 
and the peculiar tail, equally lobed on both sides of the 
prolonged vertebral column, (for which structure the name 
diphycercal has been proposed by McCoy), and supplement- 
ed by a small additional or terminal lobe, all make up an 
assemblage of characters which, more or less fully exhibited 
by these several genera, link them very closely together as a 
well-marked natural group. This peculiar type extends from 
the Mountain Limestone of Kurope to the Cretaceous, pre- 
serving its characteristic and unusual features with remark- 
able persistence, through a series of ages, and a succession of 
changes, which our minds strive in vain to grasp. 
The same persistence of type is seen at many points in the 
history of life. It is exceedingly marked in Nautilus, which 
began in the Lower Silurian, and has lived on till the present 
time, while sending out offshoots into Clymenia, Goniatites, 
and through Ceratites, etc., into the Ammonitide, all of 
which forms speedily became extinct. The Ammonites, in 
particular, sported into every conceivable variety of form, 
and frittered themselves away in excessive ornamentation; 
and after a brief, though brilliant, reign in the Mesozoic age, 
they passed utterly away. 
In reference, yet further, to fossil ganoids, it is a matter of 
much interest that species of the genera Amia and Lepidostevis 
have recently been brought to light from the Tertiary lake- 
beds of the Great Basin. The plated ganoids are all long 
extinct; while of the scaled ganoids, but seven genera have 
living representatives, five of these being American and two 
African. The scaled-ganoids have been traced from the 
Devonian to the Cretaceous, and they again appear in 
the present period: but this recent discovery of them in the 
