18 EOCENE MOLLUSC A. 



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 branchiae 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 branchise, 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: 1st, Dibranchiata, comprising those furnished with two gills; and 2d, Tetra- 

 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. ; Siphon if eres 

 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 Feron ; 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. 



