In view of the unreliability of the points of difference in rats usually given as identifying data, such as the number and location of the mammæ, the variations in color and the peculiarities of the footpads, the Javan observers depend upon the conformation of the skulls for the determination of genera, the skull of _M. rattus_ being oval and arched, that of _M. decumanus_ more closely approaching the square and rectangular conformation, and that of _Gunomys_ being broader, higher and longer than either. In _M. rattus_ the prominent borders which separate the parietal from the frontal surfaces of the skull are oval; in _M. decumanus_ they are parallel or slightly divergent; in _Gunomys_ they are lyre-shaped. [Illustration: _M. rattus_] [Illustration: _M. decumanus_] [Illustration: _Gunomys_] To determine these differences the heads of the rats are cut off, the tissues desiccated by antiformin, or by boiling and stripping. From experiences in Porto Rico, Creel, of the U. S. Public Health Service, concludes that _M. norvegicus_ (_decumanus_), while essentially a burrowing animal and not addicted to climbing or swimming, is nevertheless quite capable of doing either. He was found to burrow in the hardest earth to a depth of two and one-half feet and to pass through all kinds of wood, soft brick and lime mortar, probably by gnawing. The black rat and Alexandrine rat (_M. rattus_) in Porto Rico, according to the same observer, do not burrow at all, but can climb and jump in expert manner, and are the species found in the rural districts, remote from houses. He found that all varieties of rats may swim, from ships to the shore, distances of from one-fourth to one-half mile, but that they lack the sense of direction and probably do not land from ships naturally in this manner (_Public Health Reports_, No. 9, February 28, 1913). The female _decumanus_ is a prolific breeder and brings forth larger litters than the _Mus rattus_ female. _Mus decumanus_ is generally conceded to be larger and more ferocious than _Mus rattus_. For this reason he drives the smaller rats to the upper floors, the _decumanus_ species generally living near the ground. He is a burrower and is rarely found in the upper stories of buildings. _Decumanus_ is known as a wharf rat, but is rarely trapped on ships on the Pacific Coast, according to the observations of Surgeon Simpson of the U. S. Public Health Service (_Public Health Reports_, April 11, 1913). According to the same observer, _Mus rattus_ is the commonest ship-borne rat. He also states that the black rat and the roof rat (_Alexandrinus_), both varieties of _M. rattus_, differ chiefly in color. They live in upper floors, between ceilings, in walls and roofs and are remarkable climbers as well as being expert rope-walkers and wire-walkers. On account of their natural wariness and caution it is not always easy to induce them to enter or approach traps.