328 H. D. REED 



doubtful strand of mesenchymal tissue extending between the 

 two is to be so considered. The ossification of the fenestral 

 plate is characteristic of the whole family Plethodontidae. The 

 central portion is completely ossified while in the periphery 

 cartilage persists between the outer and inner plates of bone. 

 This is quite in contrast to the amblystomid type where an 

 inner and outer shell of bone enclose cartilage at all levels. 

 The central portion of the plate is relatively thin, and ossifica- 

 tion begins here. The reason for the thinness of the plate at 

 its center and its early and complete ossification is to be found 

 in certain developmental conditions with a morphological sig- 

 nificance, as well as the tendency in these forms to a reduction 

 of the chondrocranium beyond that found in the Amblystomidae. 

 This point will be discussed later. 



One of the characteristic features of the fenestral plate of 

 Spelerpes bislineatus is in the relation of the stylus to the plate 

 itself. As stated above, the stylus joins the plate in its cephalic 

 portion, but, even in the adult, the fusion is not so complete 

 that the identity of the former is lost. In young adults the 

 entire stylus is composed of a shell of bone surrounding a few 

 cartilage cells. In its caudal extremity, where it joins the fenes- 

 tral plate , the bony shell increases in thickness with a resulting 

 diminution of cartilage within. The stylus, after first touching 

 upon the plate, extends caudad a very short distance, in some 

 cases less than 1 mm., but it is not completely incorporated 

 with the plate substance. The cylindrical sheath of bone can 

 be made out distinctly at all levels, although an anchylosis of 

 the two structures takes place (fig. 1). At a level of 50 n caudad 

 of that of figure 1, the stylus disappears and only here does 

 there seem to be any fusion between the two elements. The 

 significance of this relation is revealed only through develop- 

 ment. The connection of the definitive fenestral plate with the 

 ear capsule is in its ventrocephalic margin where the cartilage 

 of the two structures is continuous (fig. 22) . This connection at 

 all stages is relatively narrow, and in this, as well as position, it 

 differs from any otic connection of fenestral elements in the 

 amblystomid forms where the connection is more extensive and 



