VERTEBRAE

The vertebral column consists of 7 cervicals, and ordinarily 14 thoracics, 6 lumbars, 3 sacrals and, depending on the species, 11 to 23 caudals. For the three species of which skeletons were examined, variations from the normal number of vertebrae are noted in the following table:

Table 1
Data on vertebrae in three species of the subgenus Mustela
(Numerals in parentheses indicate number of specimens)

	Mustela erminea	Mustela rixosa	Mustela frenata
Number of cervical vertebrae	7⁽⁷⁵⁾	7⁽¹²⁾	7⁽⁶⁵⁾
Number of thoracic vertebrae	14⁽⁷¹⁾	14⁽¹²⁾	14⁽⁵⁴⁾
Number of thoracic vertebrae	15⁽⁴⁾		15⁽¹³⁾
The dorsal vertebraconstituting the anticlinal	11th⁽¹⁸⁾	11th⁽¹²⁾	11th⁽⁴⁰⁾
The dorsal vertebraconstituting the anticlinal	12th⁽⁷⁾		12th⁽²⁷⁾
Number of lumbar vertebrae	5⁽²⁾		5⁽¹¹⁾
Number of lumbar vertebrae	6⁽⁷³⁾	6⁽¹²⁾	6⁽⁵⁴⁾
Number of sacral vertebrae	2⁽⁹⁾		2⁽³⁾
	3⁽⁶⁵⁾	3⁽¹⁰⁾	3⁽⁶⁷⁾
	4⁽¹⁾	4⁽²⁾
Number of pseudosacral vertebrae	0⁽⁷³⁾	0⁽¹²⁾	0⁽⁵⁷⁾
	1⁽²⁾		1⁽⁶⁾
		11⁽¹⁾
		14⁽³⁾
	15⁽²⁾	15⁽⁷⁾
	16⁽³⁾	16⁽¹⁾
	17⁽⁹⁾
Number of caudal vertebrae	18⁽²⁸⁾
	19⁽¹¹⁾		19⁽⁶⁾
			20⁽¹⁴⁾
			21⁽¹⁴⁾
			22⁽⁷⁾
			23⁽¹⁾

Variation according to the species is evident in the number of caudal vertebrae, but in the other categories of vertebrae no consistent difference in number according to species was found in the material examined. Apparently there is also some geographic variation in the number of caudal vertebrae within a species. For example, the one skeleton seen of Mustela rixosa eskimo (no. 219036, U. S. Nat. Mus., from St. Michaels, Alaska) has only 11 caudal vertebrae, whereas in the 11 Mustela rixosa rixosa from Roseau County, Minnesota, the usual number is 15 with extremes of 14 and 16. Similarly specimens of Mustela frenata from Idaho and California almost always have 1 or 2 more caudal vertebrae than do individuals of the shorter-tailed subspecies of the same species from eastern Kansas.

Of the vertebrae, only the cervicals, of which there are 7, were found to be constant in number. In M. erminea, two of the seven individuals in which the anticlinal vertebra was the 12th (instead of the 11th) had 15 instead of the customary 14 thoracic vertebrae. In M. frenata, seven of the twenty-seven individuals in which the anticlinal vertebra was the 12th (instead of the 11th) had 15 instead of 14 thoracic vertebrae. The one M. erminea with a pseudosacral vertebra had only two instead of the customary 3 sacral vertebrae but the same individual had 15 thoracic vertebrae. Of the six M. frenata with a pseudosacral vertebra, two animals had only two instead of three sacral vertebrae. Conceivably, therefore, the pseudosacral vertebra in each of the three instances mentioned may represent merely an unfused sacral vertebra, instead of a true pseudosacral as occurs in four individuals of M. frenata.