THE BATRACHIA OF NORTH AMERICA. 225 



may be exclusively air breathers I have shown by observations on a 

 specimen in an aquarium which, for two mouhs, probably from the at- 

 tacks of fishes, had no branchiae at all. (See Journ. Ac, Phila., 18G6, 

 p. 98). 



In explanation of this fact, it may be remarked that this atrophy can 

 not be accounted for on the supposition that it is seasonal and due to 

 the drying np of the aquatic habitat of the sirens. The countries they 

 inhabit arehnmid, receiving the heaviest rain-fall of our Eastern States, 

 and there is no drought. The only explanation appears to me to be that 

 the iDresent Sirens are the descendants of a terrestrial type of Batrachia, 

 which passed through a metamorphosis like other members of their 

 class, but that more recently they have adopted a permanently aquatic 

 life, and have resumed their brauchite by reversion. 



This hypothesis is confirmed by the relations of the stapes to the 

 snspensor of the lower jaw. It is not counected with the quadrate 

 cartilage, as is the case with the Protiidaj, Cryptobranchid.T, Amphin- 

 midre, and the larvae of salamanders, but is distinct and is connected 

 posteriorly with a stapedius muscle as in adult salamanders.* (See 

 PI. 4G, fig\ 5.) 



There are but two known genera of this family, which differ as fol- 

 lows : 



Digits four ; branchial fissures normally three Siren. 



Digits three i branchial fissures one Pseiidobranchus. 



SIREIST Linnajus. 



Amojnitates Academic;©, vii, 17G5, p. 311 (teste Holbrook); Systema 

 Naturse 12, ed. i, p. 371, 1766 ; Op. cit. 13, ed. i, Addenda, 1767 ; ibid., 

 Turtou'sed., 1802, i, p. 671 ; Tschudi, Batrachia, i>. 98; Gray Cat. 

 Brit. Mus., p. 68; Dam. & Bibr. Erp. Gen., ix, p. 191 ; Boulenger, 

 Cat. Grad. Brit. Mus., ed. ii, p. 86, 1882. 



Phanerohranclius, xit. Leuckart Isis von Okeu, 1821, p. 260. 



Digits four; jaws with horny sheath; tongue large, free in front; 

 eyes distinct ; external brauchitie three. A patch of teeth on each side 

 of the palate standing on three plates, which are attached to the para- 

 sphenoid bone. Three branchial fissures on each side in the adult. 



In this genus tliere are narrow cartilages on the approximated ex- 

 tremities of the first basibranchial and the ceratohyals, in the position 

 of a basihyal and hypohyals respectively. 



The transverse processes of the vertebri^e are very much expanded 

 horizontally at the base, but they terminate in a pointed apex. 



^ American Naturalist, 1883, p. 464. 

 1951— Bull 31 15 



