Mr. W. King on the Tetrabranchiate Cephalopods. 271 portion to their height, and tapering to sharper summits, which however are obtuse and bifid. The breadth of the tooth slightly increases to the posterior pair of eminences, whilst in the Mas-todon longirostris and angustidens the crown maintains the same breadth, or more commonly becomes narrower from the anterior to the posterior pair of mastoid eminences. Other differences observable on a minute comparison are too trivial to deserve notice, especially when observed in only a single example of a complex molar tooth. In the Australian specimen under consideration the mastodontal characters are unmistakeable, and the resemblance to the molar teeth of the Mastodon angus-tidens is very close. The specific distinction of the Australian Mastodon rests, at present, only on the slight differences pointed out in the form of the mastoid eminences and the contour of the crown of the molar tooth. The question may arise, whether identity of generic characters in the molar teeth of an extinct Australian mammal with those of the Mastodon can support the inference that the remaining organization of the Proboscidian Pachyderm coexisted with such a form of tooth ? The analogy of the close mutual similarity which exists in the molar teeth of the Tapir, Dinothere, Manatee and Kangaroo suggests the surmise that the mastodontal type of molar teeth might also have been repeated in a gigantic Marsu-pial genus which has now become extinct ; and such an idea na-turally arose in my mind after having received evidence of the marsupial character of the Diprotodon and Nototherium *, two extinct Australian genera, with the tapiroid type of molars, re-presented by species as large as a Rhinoceros. The more complex character of the molars of the Mastodon, and the restriction of that character, so far as is now known, to that genus only, makes it much more probable, however, that the molar here described belonged to a true Mastodon, and the species may be provisionally termed Mastodon australis. London, August 22, 1844. XXXV. — An Attempt to Classify the Tetrabranchiate Cephalo-pods. By William King, Curator of the Museum of the Na-tural History Society of Northumberland, Durham, and New-castle-up on-Tyne . The following observations on the Tetrabranchiate Cephalopods are in substance the same as some which formed part of two lec-tures which 1 delivered in the autumn of 1 841 in the Theatre of the Literary and Philosophical Society of Newcastle-upon-Tyne. * The characters of these genera and the evidences of their marsupial nature will be the subject of a future communication.
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