356 H. D. REED 



FAMILY SIRENIDAE 



The possible conclusions regarding the fenestral elements in 

 Siren are based to a minor degree upon circumstantial evidence, 

 which, however, carries weight with one who has reviewed these 

 structures in the entire series of caudate amphibia. The known 

 facts may be briefly summarized. There is only a single element 

 in the fenestra, and this is not connected with either quadrate 

 or squamosal. In individuals up to 215 mm. in length the plate 

 is connected with the ear capsule in the dorsal part of the fen- 

 estra (fig. 34). This connection is different from the cephalic 

 connection of the columella in the Amblystomidae (fig. 20) and 

 the ventrocephalic relations of the plate in the Plethodontidae 

 (fig. 22). In a specimen 440 mm. long (undoubtedly a fully ma- 

 tured individual) the fenestral plate is free from any cartilaginous 

 or bony connections with the ear capsule whatever (fig. 35). 

 Cope ('88), writing of this structure, observes: "In Siren the 

 stapes is osseus. Its columella is replaced by the stapedius 

 muscle which extends posteriorly." In view of these facts, the 

 origin and nature of the fenestral element in Siren becomes clear. 

 Its freedom from the ear capsule in the definitive state, its lack 

 of connections with the suspensorium, and the presence of the 

 M. operculare (stapedius muscle of Cope) identify it as the homo- 

 logue of the operculum of the amblystomid forms. Not only 

 does it possess the relations of the operculum, but it is formed 

 from the walls of the ear capsule by the 'cutting out' process as 

 described by Kingsbury and Reed for this element in Ambly- 

 stoma. In a specimen 133 mm. long (fig. 34) the 'uncut' por- 

 tion of the opercular margi^i is coextensive with its width, and 

 there are only slight relations with the M. operculare. Serial 

 sections of the head of a specimen 215 mm. long show the uncut 

 portion very much reduced, with the M. operculare firmly at- 

 tached to the side of the plate along with the strong ligamentum 

 hyocolumellare. 



There is no evidence as to the fate of the columella in this 

 species. In others, where this element has become functionless 

 in adult life, it has fused with the cephalic lips of the fenestra, 



