46 FRONTAL SEGMENT, OR VERTEBRA. 



this presents a joint-surface, convex in one way, concave 

 in the other, called a "ginglymoid condyle," for the hsema- 

 pophysis, or lower division of the arch. In most air- 

 breathing vertebrates — the serpent. Cut 16, e. g. — the 

 pleurapophysis resumes its normal simplicity, and is a 

 single bone, 28, which is called the "tympanic;" in the 

 eel-tribe it is in two pieces. The greater subdivision, in 

 more actively breathing fishes, of the tympanic pedicle, 

 gives it additional elasticity, and, by their overlapping, 

 interlocking junction, greater resistance against fracture; 

 and these qualities seem to have been required in conse- 

 quence of the presence of a complex and largely-de- 

 veloped diverging appendage, which forms the framework 

 of the principal flap or door, called " operculum," that 

 opens and closes the branchial fissure on each side. The 

 appendage in question consists of four bones; the one 

 articiilated to the tympanic pedicle is called "preopercular," 

 No. 34; the other three are, counting downwards, the 

 "opercular," No. 35; the " subopercular," No. 36; the 

 "inter opercular," No. 37. The hsemapophysis is sub- 

 divided into two, three, or more pieces, in different fishes, 

 suturally interlocked together ; the most common division 

 is into two subequal parts, one presenting the concavo- 

 convex joint to the pleurapophysis, and called "articular," 

 No. 29 ; the other, bifurcated behind to receive the point 

 of 29, and joining its fellow at the opposite end to com- 

 plete the hasmal arch. It is very singularly modified by 

 supporting, and having more or less firmly attached to 

 it, a number of the hard bodies called " teeth," and hence 

 it has been termed the "dentary," No. 33. In the cod 

 there is a small separate bone, below the joint of the 

 articular, forming an angle there, and called the "angular 

 piece," No. 31. 



