no COMPARATIVE MORPHOLOGY OF VERTEBRATES 



hard palate, but in some edentates and whales the pterygoids form part of the 

 partition between the narial passages and the mouth cavity. 



The ingrowth of the hard palate has forced the vomer from the roof of the 

 mouth to a position just ventral to the anterior part of the cartilage of the 

 nasal septum. In the monotremes there is a 'dumb-bell bone* in front of the 

 vomer (p. 77), A lacrimal bone always occurs at the inner side of the orbit 

 and the zygomatic forms the external wall of that cavity. 



The lower jaw articulates directly with the squamosal without the inter- 

 vention of a quadrate (see Ear Bones, p. 80). Its halves may unite in front by 

 ligament or by complete anchylosis. It is usually described as consisting of a 



pair of dentaries, but there are several 

 centres of ossification and a splenial 

 and possibly a coronoid element may 

 be recognized. The angulare is ap- 

 parently the tympanic, while the 

 articulare of lower vertebrates is the 

 malleus. A remarkable feature in 

 development is an enormous cartilage 

 Fig. iis.-Hyoidof rhino^^^^^./- ^^ ^^^ posterior angle of the jaw, the 

 odus). ac, anterior cornu; b, body; c, dorsal side of which forms the condyle 

 ceratobyal; e, epihyal; pc, posterior for articulation with the glenoid fossa. 



cornua (thyrohyal). t*^ u 1 • .. • 



■^ -^ Its homologies are very uncertam. 



The hyoid apparatus varies. As described above, the hyoid is connected 

 above with the otic region, below with the first branchial. The part con- 

 nected with the otic capsule forms the styloid process (p. 109), while the rest 

 may ossify as epi-, cerato-, and hypohyals, or a part may change to a stylohyal 

 ligament, connecting the ventral parts with the skull. The hyoid of the adult 

 (fig. 115) consists of the copula forming the body, a part of the hyoid the an- 

 terior cornua, while the first branchial arch (of which at most but one or two 

 'thyreohyal* elements are formed) give rise to the posterior cornua. These 

 are connected by ligament with the greatly modified posterior branchial arches, 

 described in connexion with the larynx (see respiratory organs). 



Appendicular Skeleton 



The appendages fall in two categories, the median or azygos 

 (median fins) found only in aquatic vertebrates, and the paired appen- 

 dages, which (cyclostomes excepted) are found in every class, 

 although here and there individual species or genera may lack 

 them. Both kinds have an internal skeleton. Opinions dififer as 

 to the origin of these appendages. The two most prominent views 

 are given below. 



According to one view the two types have no relation to each other. The 

 paired appendages are derived from gill septa, all traces of which are otherwise 

 lost from these somites. The girdles which support the appendages are modi- 

 fied gill arches, while the skeleton of the appendage itself is derived from the 



