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224 IJULLETIN :«l, UNITED STATES NATIONAL MIKSEKM. 



is pii'st'iit as ii ciirtiliij,'! HOIKS baiul ; the prefrontal boiios arc waiitiiifr; 

 tlu'orbitosplu'iioids are lar^'e ami e.\i»aii(li'<l laterally in front, iso as to 

 form iiart of tiie palatal surface. The earpns is eartila;;inons, anil tlieie 

 are no hind le^s or pelvic arch. There are external branchia', which 

 consist of branchinjr processes of the inte^ninient of the epibranchial 

 elements. The latter are separated by branchial lissnres of the walls 

 of the j)harynx. 



In the jjenns Siren the cranial extremity of thoceratohyal is free from 

 the cranium, but is connected with the stapes by a stronj^ lijiament. In 

 this resi)ect this };enus resembles the adults of the true salamanders, oi' 

 rsendosauria, rather th an the other perennibranchiate forms, <»i- the Tre 

 motodera and Amphiumoidea. In its four epibranchial ('intilaj^es, how- 

 ever, if resembles the larva* of the Psendosauria, as also in the presence 

 of a second basibranchial, connected with i\n', first anteriorly, and ex- 

 pandiiif; posteriorly Tins mixture of charatiters of the adults and (»f 

 the larva' of pseiulo'aurian urodela has asi^nilicance which I will fur- 

 ther illustrate. 



1 have already pointed out (American Naturalist, 188,1, p. 215) that 

 l»ala'oiitolo}jfy shows that the order of Trachystomata is a de;;i'iicrate 

 type, if the structure of its skull, limb-arches, and limbs be I'onsidcrcd. 

 1 have also reason to believe that there are indications of a retrojiiade 

 metainorphosis to be found in the history of its branchial apparatus. I 

 was for a long time at a loss to account for the i'lirioiis condition which 

 1 had observed in the branchia' of the sirens. The fringes are freipiently 

 in a state of apparent partial atrophy and inclosed in a common dermal 

 investment of the branchial ramus, or all the rami are covered l»y a 

 common investment, so as to be absolutely functionless and immovabU^ 

 This character observed in the Pucudohnnnltus striutiis, iiiiva orifjin to 

 its separation from the {jeniis Siren. The character is, however, comnioii 

 to the Siren lacertina at a certain age, and the real diH'erence betwt'cn 

 the genera depeinls on thedilferent number of the digits and pharyn- 

 geal tissnres in the two. 



1 have also observed that the liinctionless condition of the branchia' 

 is universal in young individuals of the tSiren lacertina of live ami six 

 inches in length, and that in a specimen of a little over three inches 

 they are entirely rudimentary and subepidermal. 1 have, in fact, no- 

 ticed that it is only in large adult specimens that the branchia' are fiiby 

 developed in structure and function. The inference from the specimens 

 certainly is that the branchia' are in the sirens not a larval character, 

 as in other perennibranchiate Batrachia, but a character of maturity. 

 Of ct)urse only direct observation can show whether sirens have 

 branchia' on exclusion from the egg; but it is not probable that they 

 dili'er so much from other members of their (ilass as to be without them. 

 Nevertheless, it is evident that the branchia' soon become functionless, 

 so that the animal is almost if not exclusively an air breather, and that 

 functional activity is not resumed till a more advance<l age. That Sirens 



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