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 Didelphide. 
The genus Amphitherium, 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. ‘T'wo species are already described. The 
genera Amphigonus and Heterotherium 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 Blainviile, 
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. Tuytactnipa. Includes but one genus, Thylacinus, with a sin- 
ole 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 closely 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 mamme. 
The caves of Wellington Valley have yielded remains of one species of 
this family, T. speleus. 
Orper 38. Creracea. 
The order of Cetacea is one whose history is still veryincomplete. Although 
comprising the largest 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 assign to it, is characterized by a naked and smooth 
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