260 MARSUPIALIA. 



the cortical and tubular parts ; the single mammilla has one sharp ridge, 

 with hardly any lateral abutments. The capsula renalis is small. 



The heart is a regular rounded body, without much distinction 

 externally between the right and left sides: the auricles are smooth 

 and rounded at the edges, which correspond with the shoulders of the 

 ventricles. There is a vena cava superior sinistra. The pericardium 

 is attached to the large vessels near to the heart. The mediastinum is 

 pretty broad. 



The lungs on the right side have three lobes, the middle one small, 

 and the most detached being something like the lobulus medius : the 

 lungs on the left side have two lobes, or one lobe with a deep fissure. 

 The lobulus medius is of a triangular form ; the lower and posterior 

 edges are attached by a thin membrane to the oesophagus 1 . 



The Opossum [Didelphis virginiana, Did. Opossum, and Did. 

 dorsigera, Linn.] . 



This animal is distinct from all others, so far as I know. There is no 

 animal that it can be immediately classed with ; excepting it be a second 

 degree from the monkey, mocock, mongoose, and sanguine ; considering 

 them as the first steps from the monkey, and this as the second. 



It would appear that opossums paired, for an old male paid great 

 attention to two young ones that were put to him. They ate fruits of 

 all kinds. 



Mr. Smeaton, from Virginia, who brought me an opossum, told me 

 that he had two brought to him by a negro, one of which was a female 

 with young in the pouch, which were smaller than any young mouse 

 he ever saw, with no hair on them 2 . This was in the month of Febru- 

 ary ; therefore they must breed in the winter. 



1 [The tongue and larynx of the Phalangi&ta vulpina form No. 1504; the male 

 organs are Nos. 2475, 2480, Physiol. Series.] 



2 ["I have been so fortunate as to ascertain the size and weight of several embryons 

 immediately after their exclusion from the uterus. One of them weighed only one 

 grain ! The weight of each of the six other young ones was but little more than this. 



" The young opossums, unformed and perfectly sightless as they are at this period, 

 find their way to the teats by the power of an invariable, a determinate instinct ; . . . 



" In this new domicilium they continue for about fifty days ; that is until they 

 attain the size of a common mouse (Mus musculus), when they begin to leave the 

 teats occasionally, but return to them again, until they are nearly the size of rats. 



" .... At the end of about fifty or fifty -two days from its first reception in the 

 pouch the eyes of the young begin to open. 



" I have found that the same embryon has increased in weight 531 grains in sixty 

 days ; that is, at a rate of almost nine grains daily. . . . The animal attains to nearly 

 its full growth in about five months ; but never, I believe (in our latitudes I mean), 

 procreates the first year of its existence. 



" On the 21st of May, upon looking into the box which contained the female 



