MAMMALIA. 419 



(C. variegatus), characterized by large hind feet, the toes of which are 

 united by a web ; the fore feet moderate, and with an unusual elongation 

 of the pisiform bone. Females are provided with a perfect pouch. The 

 ears are large and naked ; the tail is longer than the head and body taken 

 together ; the fur is dense, short, and somewhat woolly. Its habits are 

 aquatic. It is found in Guinea and Brazil. 



The oldest representatives of the order Marsupialia, found in the oolite 

 of Stonesfield (England), belong to the family of Didelphidas. 



The genus A?np hither ium, or Thylacotherium, as both names are used, 

 differs from Didelphis by its molars, which are more numerous and smaller ; 

 and from Myrmecobius in having proportionally larger teeth. We know 

 only the lower jaw, which has six incisors, one moderate canine, six premo- 

 lars, and six tricuspid molars. Two species are already described. The 

 genera Amphigonus and Heterotheinum are other denominations by which 

 the same remains have been designated. 



The genus Phascolotherium resembles Didelphis still more closely, since 

 it has but three premolars and four molars. The form of the teeth them- 

 selves has something of that of Myrmecobius. One single species is 

 known. 



The genus Pterodon we mention here, although, according to Blainville, 

 it comes nearer Dasyurus. It is known by a fragment of a lower jaw, from 

 the gypsum of Montmartre, in Paris ; its true affinities are still a matter of 

 doubt. 



Fam. 3. Thylacinid^. Includes but one genus, Thylacinus, with a sin- 

 gle living species, T. cynocephalus, or dog-faced opossum, about equal in 

 size to the wolf The form of the head is like that of a dog ; the tail about 

 half the length of the body ; the fur is short, and closel}^ applied to the skin. 

 Inhabits Van Diemen's Land. 



The premolars are more numerous in Thylacinus than in Dasyurus, there 

 being three of these teeth on either side of each jaw. The canine teeth are 

 of large size, of a simple, elongated, conical form, and slightly recurved at 

 the apex. The marsupial bones are wanting. The female is provided with 

 a distinct pouch and four mammae. 



The caves of Wellington Valley have yielded remains of one species of 

 this family, T. spelceics. 



Order 3. Cetacea. 



The order of Cetacea is one whose history is still very incomplete. Although 

 comprising the lai'gest of all mammalia, it belongs to an element in which 

 science is often powerless, and thus naturally escapes our investigation. 

 Cetacea are by no means scarce, but their large size prevents them from 

 being preserved complete in our collections, so that we have no materials 

 for comparison. Occasionally a skull, a jaw, a rib, or a vertebra, are the 

 only data on which we can base our researches. This order, restricted 

 within the limits we as-sign to it, is characterized by a naked and smooth 



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