Baiomys musculus

Abstract

Baiomys musculus (Merriam, 1892) Southern Pygmy Mouse Sitomys musculus Merriam, 1892:170. Type locality Colima, Co-lima. Baiomys musculus Mearns, 1907, first use of current name-combination. CONTEXT AND CONTENT. Order Rodentia, Family Cricetidae, Subfamily Cricetinae. The genus Baiomys contains two recent species and six extinct species. Eight subspecies of Baiomys musculus are currently recognized (see Packard, 1960:610) as follows: B. m. brunneus (J. A. Allen and Chapman, 1897:203). Type locality , Jalapa, Veracruz. B. m. grisescens Goldman, 1932:121. Type locality, Comyabuela [Comayagiiela], just south of Tegucigalpa, Honduras. B. m. handleyi Packard, 1958:399. Type locality, Sacapulas, El Quiche, Guatamala. B. m. infernatis Hooper, 1952:96. Type locality, Teotitlan, Oa-xaca. B. m. musculus (Merriam, 1892:170), see above. B. m. nigrescens (Osgood, 1904:76). Type locality, Valle de Com-itan, Chiapas. B. m. pallidus Russell, 1952:21. Type locality, 12 km NW Axochiapan, Morelos, (nebulosus Goodwin, a synonym). B. m. pullus Packard, 1958:401. Type locality, 8 mi S Condega, Esteli, Nicaragua. DIAGNOSIS. Size is large for the genus (extremes in external measurements of adults); total length is 100 to 135, length of tail vertebrae, 35 to 56, length of hind foot, 14.1 to 17, and length of ear, 9 to 12. Baiomys musculus may be confused easily with B. taylori (only adults and old adults fit this diagnosis); hind foot is 16 mm or more; occipitonasal length is 19 mm or more; zygomatic breadth is 10 mm or more; rostrum is not deflected ventrally at frontoparietal suture but, instead, curves gradually toward an-teriomost point of nasals; cingular ridges and secondary cusps on teeth are pronounced; basihyal of hyoid apparatus has anterior point with entoglossal process, shoulders of basihyal protrude anteriorly (characteristic of all age categories); baculum has broad shaft with spatulate to knob-shaped tip, wings at base project anteriorly ; baculum is more than 3 mm long; short process of incus is knob-shaped rather than attenuate; muscular process of posterior crus of stapes is prominent. Dentition is 1/1, 0/0, 0/0, 3/3. The molars of B. musculus have more pronounced cingular ridges than those of B. taylori and are somewhat more hypsodont. GENERAL CHARACTERS. Mean and extremes of external measurements (mm) of eight adults of B. musculus musculus (largest subspecies) from Armeria, Colima (Packard, 1960:621), are: total length, 125.5 (115 to 135); length of tail, 47.5 (42 to 54); length of body, 75.6 (69 to 81); and length of hind foot, 16.5 (16 to 17). Means and extremes of selected cranial measurements in millimeters of above adults are: occipitonasal length, 20.3 (19.8 to 20.7); zygomatic breadth, 10.7 (10.3 to 11.1); postpalatal length, 7.4 (7.1 to 7.7); least interorbital breadth, 4.0 (3.9 to 4.1); length of incisive foramina, 4.3 (4.1 to 4.5); length of rostrum, 7.3 (6.9 to 7.6); breadth of braincase, 9.8 (9.4 to 10.0); depth of cranium, 7.1 (6.7 to 7.2); and alveolar length of maxillary toothrow 3.4 (3.3 to 3.6). The southern pygmy mouse varies in color of upper parts from dark reddish brown or ochraceous-buff to nearly black; un-derparts are pale pinkish buff to white or pale buffy. Young mice are more grayish than adults and lack colors distinctive of the described subspecies. There is little secondary sexual variation (see Packard, 1960). There are three distinct pelages-juvenal, postjuvenal, and adult. The uniformly dusky gray juvenal mice begin their molt at about day 38 to 45 with the appearance of new brownish hair on the top of the head, and complete this molt by day 52 to 60. The molting sequence is similar to that of Peromyscus (for a detailed analysis of molt, see Packard, 1960).