II SKULL 29 



is scarcely developed at all, for instance in Bnfo. la Pelohilrfi 

 cidtripcs the squamosal is very wide and forms a junction with 

 the fronto-parietals, thus producing a broad l)ridge across tlir 

 temporal fossa. 



The nasal bones are large and meet in the middlf line. 

 Frequently they leave a space between them and the; diverging 

 anterior portion of the fronto-parietals, through which gap 

 appears part of tlie dorsal surface of the ethmoid cartilage. 

 A tbntanelle between the frontals occurs in most Hylidae, many 

 C'ystignathidae, some few Bufonidae, in Prlochjtes amongst tlie 

 Pelobatidae, and in the Discoglossidae. 



The tympanic cavity is bordered in front, above, and below 

 by the squamosal and quadrate, behind by the musculus depressor 

 mandibulae, internally by the otic capsule, and by the cartilage 

 of the cranium between this and the lateral occipital bone. 

 Tlie cavity communicates, however, b}- the wide and short 

 Eustachian tube with the moiitli, the passage being bordered 

 anteriorly by the pterygoid, posteriorly by soft parts. Partly 

 imbedded in these soft tissues is the styloid process or stylohyal, 

 which is attached to the cranium, mostly behind the otic region, 

 and is continued downwards into the anterior horn of the hyoid. 

 The whole partly cartilaginous, ligamentous, and osseous string 

 is, in fact, the entire ventral half of the hyoid arch, while the 

 dorsal half or hyomandibular portion of this, the second visceral 

 arch, is modified into the columellar or auditory chain. The 

 inner end of this chain, the stapes, is inserted into and around 

 the fenestra ovalis of the otic capsule, while the outer end is 

 somewhat T-shaped, and is loosely attached to or near the upper 

 rim of the tympanic ring and to the middle of the tympanic 

 disc. In many Anura this terminal bar can be seen from the 

 outside. The middle portion of the columellar chain is ossified, 

 the rest remains cartilaginous. But the whole chain exhiljits 

 various modifications in different genera, especially in the 

 number and the extent of tlie processes sent out by the outer 

 cartilaginous portion ; these are attached in various ways to 

 the tympanum and its rims. Tlie tympanic disc is carried 

 by a cartilaginous ring, which rests against a special process 

 sent out by the (|uadrate, and is probably itself a differentiation 

 of this element. 



In some very aquatic genera the whole tyiiqwniic cavity is 



