18 EOCENE MOLLUSCA. 
will find a most comprehensive and able review of the progress of this branch of 
natural history. 
The principle of classification adopted by Cuvier removed many of the difficulties 
and inconsistencies which had previously prevailed; but it was still based, to a great 
extent, on external characters. Attempts at arrangements, founded on higher 
characters, were made by different authors; but the imperfect knowledge which 
existed of the anatomy of the animals, prevented the establishment of a system in 
which due regard could be paid to affinities indicated by internal organization. Of 
late years, however, considerable additions have been made to our knowledge of the 
anatomy of these animals; and in 1830, the arrival in this country of a specimen of 
the pearly Nautilus, caught off the coast of one of the New Hebrides, enabled 
Professor Owen to examine the internal structure of that animal, an opportunity which 
had not occurred to naturalists since the time of Rumphius. The anatomy of various 
other Cephalopods was also investigated by Professor Owen; and the additional 
information thus obtained, led that gentleman, in 1836, to propose a system of classifi- 
cation which, although at variance in many respects with all previous arrangements, 
was at once received as one founded, in its general principles, on well-defined and 
natural characters; and this system, accordingly, forms the basis of the more recent 
classifications.* 
All the Cephalopods the anatomy of which had been examined previously to the 
arrival of the pearly Nautilus, respired by the agency of two branchie or gills, 
and possessed three hearts, a systemic heart, and two lateral hearts; they were 
also endowed with eight arms furnished with suckers, some genera having also two 
elongated tentacula or additional arms. The pearly Nautilus, however, was found to 
be possessed of four branchiee, and of only one heart; and, instead of arms, the mouth 
of the animal was surrounded by numerous short tentacula. Availing himself of these 
natural and well-defined characters, Professor Owen divided the Cephalopoda into two 
orders: lst, Dibranchiata, comprising those furnished with two gills; and 2d, Zetra- 
branchiata, comprising those furnished with four gills. The Dibranchiata were 
subdivided into two sub-orders or tribes, according to the number and condition of 
their locomotive organs; the first tribe (Octopoda) consisting of the Cephalopods with 
eight arms, having the suckers simple, and the branchial chamber divided by a 
diaphragm; the second tribe (Decapoda) consisting of those Cephalopods possessed 
* Up to this time Spirula, as well as Belemnites, had been classed with Nautilus, and the other 
Cephalopods which now form the tetrabranchiate order (Ceph. test. polythalamaces of Lam. ; Siphoniferes 
of D’Orb.) Of the anatomy of the animal nothing was known; but the presence of an ink-bag, and the 
acetabuliferous character of the arms had been shown by Lamarck and Peron; and from this fact Professor 
Owen, aided by that knowledge of the laws of correlation which imparts such value to all his observations, 
inferred that the animal must present the dibranchiate type of structure. The accuracy of this deduction 
is now fully established. 
