5Q ANATOMY OE VERTEBRATES. 



the inner one below, whence the membrane is continued to the 

 basisphenoid, paroccipital, and round by the mastoid to the tym- 

 panic again. A part of the periphery of the drum may show an 

 epiphysial bony rim. In some birds there is a well-defined flat 

 oval surface on the outer side of the pedicle for a corresponding 

 surface on the mastoid process : ' most show a distinct articular 

 surface^ on the inner side of the lower part of the base of the 

 orbital plate for the pterygoid : thus, including the squamosal pit 

 and two mandibular condyles, there may be not fewer than seven 

 articular surfaces in the tympanic bone of the Bird. Its orbital 

 process is a greater developement of the anterior lamina of the 

 Crocodile's tympanic, fig. 93, 28, vol. i. ; the size of the process 

 is one of the chief characteristics of the tympanic in the Bird, and 

 shows much variety of shape and proportion in the class.^ Its 

 apex may be truncate {Dichis),'^ or rounded (^Dinorids), or pointed 

 (Aquila). A large pneumatic foramen may be situated on the 

 inner side of the pedicle; or on the hinder facet below and be- 

 tween the upper condyles, or in both situations. 



The mandible or lower jawbone is ossified usually from nine 

 centres ; the anterior being the first to appear, forming the 

 chief and characteristic part, fig. 25, H. iii, of the bone. It 

 bifurcates as it extends backward to form the homologues of the 

 dentary elements, fig. 31,32, which are thus ' connate ' at their 

 symphysis. The Pelicans are an exception, and exemplify the 

 normal separate ossification of each dentary, becoming subse- 

 quently confluent for a small extent anteriorly.^ The ^suran- 

 gular,' ib. 29^ speedily unites, if it be not connate, with the 

 * articular,' 29 : the angular, so, remains longer distinct, but co- 

 alesces first with the articular : the splenial element, 3i, coalesces 

 first mth the dentary, and retains longest its primitive indepen- 

 dence posteriorly. 



In the Garefowl (^Alca impennis), each dentary retains its bi- 

 furcate hind end distinct, the upper prong overlapping the sur- 

 angular, the lower one the angular ; and these two latter elements 

 are divided by an oblong space partly closed within by the sple- 

 nial : there is also a foramen at the back part of the surangular. 

 The splenial retains its distinctness posteriorly, and a groove on 

 the lower margin of the ramus indicates the extent of its for- 

 ward production to its confluence with the dentary. A vacuity 

 between the angular and surangular remains in many birds, 



' xvr. vol. iii. p. 3.56, pi. 53, fig. 9,/. ^ lb. fig. 10, g. 



^ Compare ib. pi. 39, figs. 7, 8, 9, a, and pi. 53, figs. 8, 9, k. 

 * Ib. vol. iii. p. 35. ^ xxiii*. 



