The Smooth Facial Muscles of Anura and Salamandrina. 331 



The development of a mechanism to take the place of muscles 

 in closing and opening the external nares in Anura has involved 

 profound changes of structure in the entire rostral region, including 

 the lower jaw, by which the mechanism is controlled. Curiously 

 enough, however, the modification of the latter has much increased 

 the difficulty of closing the mouth, to overcome which special adapta- 

 tions are introduced. The nature of these modifications and their 

 significance will now l)e considered, beginning with the closing of 

 the mouth. 



1. The mechanism for Closing the Mouth in Rana. 

 a. The Structure of the Jaws. 

 The lower jaw of Rana is distinguished from that of the sala- 

 mander, first of all, by its rod-like form and an entire absence of 

 teeth and lips (see Fig. 10, PI. XVII). In Rana the surface which 

 comes in contact with the upper jaw is overlaid by smooth epithelium 

 which is directly continuous with that of the outer skin. The cutis 

 of the lateral surface of the lower jaw is supplied with numerous 

 glands similar to those of the skin. The bony support of the jaw 

 includes, in each half, an angulare and a dentale, each of which 

 incloses a remnant of Meckel's cartilage. Anteriorly each dentale 

 presents a short segment, pars mentalis of the dentale [P.m.d.^ Fig. 10, 

 PI. XVII), which is articulated to the longer posterior portion by a 

 free syndesmosis. In the median line the union of the two partes 

 mentales is effected by Meckel's cartilage, which here extends out 

 of the bony sheath of the dentale and forms on each side a slight 

 enlargement [Mck., Fig. 10), which is joined by means of connective 

 tissue to its fellow of the opposite side. At the point of union of 

 the two halves of the lower jaw there is formed in this manner a 

 median tubercle (tuberculum praelinguale). Lateral from this tubercle 

 lies on each side a depression (fossa praeliugualis), corresponding to 

 the short body of the pars mentalis. Still farther caudalward begins 

 the posterior segment of the dentale with a slight prominence. A 

 cross section of the dentale has the form of an ellipse, the dorsal 

 end of whose major axis is inclined somewhat toward the middle 

 line. The ventral end of this axis is therefore directed toward the 

 upper lip, which, by reason of its inward curvature, closely embraces 

 the lower jaw at this point. Along this lateral surface of the den- 

 tale occur the glands already mentioned. 



Morpholog. Jahrbucli. 2!t. 23 



