396 BULLETIN 34, UNITED STATES NATIONAL MUSEUM. 



I. Heel of extended bind limb reacbing to or beyond extremity of muzzle ; vomerine 

 teeth entirely between choanie. (No black ear-patch.) 



Dorsal dermal plicae thin, usually more than two between the lateral ones; 



spots usually smaller, rounded ; males with vocal sacs M. virescens. 



II. Heel of extended hind limb not reaching end of muzzle ; vomerine teeth between, 

 but projecting posterior to posterior border of choanse. (No black ear- 

 patch.) 

 a. Males with an external vocal vesicle. 



A dorsolateral dermal fold, with others between ; three phaUnges of fourth 

 toe free from web ; legs cross-barred B. areolata. 



Dorsolateral fold only; two phalanges of fourth toe free from web; heel 

 scarcely reaching tympanum, which is nearly as large as eye ; brown, "with 



small gray (or in life green) spots B, montezumw. 



aa. No external vocal sacs in males. 



Four dermal dorsal ridges; two phalanges of fourth toe free ; colors pale, 

 with rows of large quadrate brown spots B. palastria. 



Dermal fold, size of tympanum, and extent of web variable; length not ex- 

 ceeding 6 centimeters; large dark spots on back B. septentr ion alia. 



A dorsolateral dermal fold ; web leaving two phalanges of fourth toe free, 

 no dorsal spots; size not exceeding 8 centimeters B. clamata; 



No dorsolateral dermal fold ; web generally leaving one phalange of fourth 

 digit free ; length reaching 20 centimeters B. catesMana. 



III. Heel not reaching end of muzzle ; vomerine teeth behind choause. (A black ear- 



patch.) 



Usually two phalanges of fourth digit free ; internal cuneiform tubercle small 

 .B. temporaria. 



Usually three phalanges of fourth digit free; internal cuneiform tubercle 

 large and prominent; middle of back rarely spotted; small, size 5 centi- 

 meters B. cantabrigensis. 



IV. Heel reaching or exceeding end of muzzle, vomerine teeth behind choanse. (A 



black car-patch.) 

 cc Tympanic disk distinct. 



Head short, obtuse, entering length 3.5 times; third phalange of fourth toe 

 bordered by web and last two free ; cross-bands of tibia imperfect or 

 very few, dorsal spots small (in American subspecies) B. agilis. 



Head obtuse, entering length about three times; palmatiou bordering pe- 

 nultimate phalange, leaving the last one free; dorsal spots large ; size 

 large, reaching 12 centimeters. B. draytonl. 



Head acute, three times in length ; back without large spots ; web border- 

 ing antepenultimate phalange, leaving two free ; small, length 5 centi- 

 meters B. silvatica. 



aa. Tympanic disk concealed. 



Head obtuse ; paimation extending to base of last phalange ; skin thick, 

 glandular; size small B. toijlii. 



It may be seen from the above table that the species of Rana found 

 in ]!^orth America are closely related, and that their discrimination re- 

 quires close attention. More or less numerous exceptions to the defini- 

 tive characters above given exist, and increase the difficulty of distin- 

 guishing tbem. Thus the hind legs of the Kana palustris are of variable 

 lengths, about half the specimens having them long, as in B. virescens. 

 In the latter, the vomerine teeth in the subspecies Brachycephala fre- 

 quently are placed a little posteriorly, as in the E. palustris. The latter 

 again sometimes has four dorsal ridges, as in B. virescens. Sometimes 



