III. The Osteology and Myology of Didelphys Virginiana. By Elliott Coues. 

 With an Appendix on the Brain. By Jeffries Wymak. 



Kead October 6th, 1869. 

 INTRODUCTORY. 



k^OME account of the general traits of Didelphys Virginiana, its external form and 

 covering, and dentition, may properly precede, and serve as an introduction to, the 

 study of its bones and muscles. Among other articles ujjon its anatomy, physiology, 

 and zoology, those cited below may be consulted. ■* 



The present memoir is based upon original dissections and preparations, from which 

 all the descriptions have been taken, except in a few instances where the contrary is 

 stated ; and all the illustrations were drawn by the writer from the same sources. 



The opossum's form is adapted to the execution of numerous and varied movements, 

 but not fitted for great strength, sustained effort, or rapid progression. The animal is 

 sluggish, usually moving slowly and deliberately ; its fastest pace is a kind of amble, when 

 both legs of the same side are simultaneously moved, as is also the case when it is walk- 

 ing more slowly ; it has not been observed to trot or lope. It is plantigrade, and the 

 body is usually carried so low as to barely clear the ground. The body is thickset and 

 clumsily shaped ; the centre of motion is behind the middle, as shown by the general 

 contour, as well as by the trend of the spines of the vertebrae. Extension of the body, 

 as a whole, is less conspicuous than flexion, and is not ordinarily exhibited except when 

 the animal is hanging suspended by the tail ; but the body may be bent double, or rolled 

 in a ball, with the greatest ease, the fore and hind feet passing by each other at the mid- 

 dle of the belly, and the nose pointing backward, reaching the root of the tail. The 

 faculty of bringing the fore-paws and snout into such unusual positions is undoubtedly 

 in subservience to the necessity of certain vokmtary acts on the part of the female during 

 and immediately after the act of parturition, such as the removal of the helpless foetus to 

 the pouch, and subsequent fostering of it. Great power of flexion is also essential to ready 

 recovery of an ordinary position after suspension by the tail ; as may easily be shown by 

 holding up one of these animals in this position, when it will be observed to bend the 

 neck and shoulders strongly forward, reach upward with its fore-paws until it can catch 



' Owen. Article Marsupialia: Cyclopedia of Anatomy North America, li, 107, pi. 66. 1851. (Description and 



and Physiology, iir. (1841.) Natural History.) 



Meigs. American Philosophical Society. April, 184 7. Baird. Mammals of North America, 232. 1857. (De- 



(Reprodiiction.) scription, etc. See other authors there quoted, for Zoology, 



BACii:\r.A.N. Proceedings Academy N.nturnl Sciences, etc.) 



Philadelphia, pp. 40, 46. April, 1848. (Reproduction.) Owen. Anatomy and Physiology of Vertebrates, li, and 



AuDunON AND Bachman. Viviparous Quadrupeds of in, passim. (1866-68.) 



MEMOIRS B09T. BOC. NAT. HIST. VOL. II. y*' ^' ^^ ^^ 11 



