RODENTS OF LIBYA 205 



Diagnosis. Two distinct color types of Mus musculus occur in 

 Libya, a light-bellied form and a dark-bellied form. In the light- 

 bellied form, the hairs of the venter are either white throughout or 

 white distally with plumbeous bases. The dark-bellied form has gray 

 or brown-tipped hairs on the venter, which are always plumbeous 

 basally. The color of the dorsal and ventral pelage is markedly con- 

 trasting in the white-bellied form, whereas, in the dark-bellied form, 

 the color of both surfaces is quite similar. The two forms differ ap- 

 preciably in the color of the dorsum, that of the dark-bellied form 

 being markedly and more uniformly darker (ranging from Saccardo's 

 Umber to Sepia) ; in the light-bellied form, it is more variable and 

 ranges from Clay Color to Olive Brown and frequently is variegated 

 owing to suffusions of gray and buff. The dorsal surfaces of the hind 

 feet are dusky or gray in the dark-bellied form, whereas they are 

 almost white in the light-bellied form. In both forms, the tails are 

 naked except for a scanty covering of inconspicouus whitish hairs. 



Cranially, the two types do not differ significantly and both have 

 extremely small, gracile skulls with flattened braincases, short rostra 

 (more pronounced in the white-bellied form), large anterior palatine 

 foramina, small posterior palatine canals, relatively small cheek teeth, 

 broad basioccipitals, and small, bulbous auditory bullae. The "Mus 

 notch" is pronounced on the upper incisors. 



Remarks. The systematic status of the house mouse (Mus 

 musculus) has never been firmly established. The principal effort to 

 resolve the nomenclatural relationships of Mus musculus is that of 

 Schwarz and Schwarz (1943), who considered the wild species, Mus 

 musculus, to originally consist of four subspecies, M. m. wagneri 

 Eversmann, M. m. spicilegus Pet6nyi, M. m. manchu Thomas, and 

 M. m. spretus Lataste. According to the above authors, these wild 

 forms (except for M. m. spretus, which occurs exclusively in the wild 

 state) have given rise to the various commensal forms of the house 

 mouse throughout the world. Schwarz and Schwarz contend that most 

 of these commensal forms, except those of western Europe, Russia, 

 and Japan, have been derived from M. m. wagneri, which developed 

 both an eastern and western series of commensals. The commensal 

 Mus musculus of Libya supposedly arose from the western component 

 of M. m. wagneri, which had its origin in southwestern Persia ( = Iran) 

 and from which it spread, by the agency of man and cultivation, 

 through Iraq, Turkey, Syria, Israel, the Nile Valley, and westward 

 across North Africa. From Africa these commensal forms have reached 

 Italy, Spain, and western Europe. 



Throughout most of their range, depending upon the degree of 

 specialization, these commensals form two types, a more specialized 

 "commensal" from and a more generalized "wild" form. The com- 



