482 



MANDIBVLAR AX J) IIYOTJ) BARS. 



stomata ; and that the peculiar liistoiy of this arcli in the Tadpole is a 

 more or less true record of its phylogenetic develojmient. In favour of this 

 view is the striking similarity which Huxley has jjointed out between 

 the oral skeleton of the Lamprey and that of the Tadpole; and certain 

 peculiarities of the mandibular arch of Chimsera and the Dipnoi can perhaps 

 best be explained on the supposition that the oral skeleton of these forms 

 has arisen in a manner somewhat similar to that in the Frog ; though with 

 reference to this point further developmental data are much required. 



On the other hand the above sup|iositions would necessitate our 

 admitting that a great abbreviation has occurred in the development of 

 the mandibular arch of the otherwise more primitive Urodela ; and that 

 the simple mode of growth of the jaws in Elasmobranchii, from the 

 primitive mandiltular ai-ch, is phylogenetically a much abbreviated and 

 modified process, instead of being, as usually supposed, a true i-ecord of 

 ancestral history. 



If the view is accepted that the characters of the mandibular arch of 

 the Tadpole are secondary, it will be necessary to admit that the adaptation 

 of the mandibular arch to the suctorial mouth took place after the suctorial 

 mouth had come to be merely a larval organ. 



In view of our imperfect knowledge of the development of most Piscine 

 skulls I would refrain from expressing a decided opinion in favour of 

 either of these alternatives. 



As the tail of the Tadpole gradually 



Ftg. 339. Tadpole with tail beginning to 

 shrink ; side view of skull without the bran- 

 CHIAL AKCHEs. (From Parker.) 



n.c notochord ; au. auditory capsule ; between 

 it and eth. the low cranial side wall is seen ; eth. 

 ethmoidal region ; st. stapes ; 5. trigeminal fora- 

 men ; 2. optic foramen ; ol. olfactory capsules, both 

 seen owing to slight tilting of the skull ; c. tr. cornu 

 trabecule; u.l. upper labial, in outline; su. suspen- 

 sorium (quadrate) ; pd. its pedicle ; ot.pr. its otic 

 process; or.p. its orbitar process; t.m. temporal 

 muscle, indicated by dotted lines passing beneath 

 the orbitar process; pa.pg. palatopterygoid bar; 

 vik. Meckelian cartilage ; l.l. lower labial, in out- 

 line; c.h. ceratohyal; h.h. basihyal. The upper 

 outline of the head is shewn by dotted lines. 



disappears, and the meta- 

 morphosis into the Frog 

 becomes accomplished, the 

 mandibular arch undergoes 

 important changes (fig. 

 339) : the palato-pterygoid 

 attachment { pa.pg) of the 

 quadrate subocular bar be- 

 comes gradually elongated ; 

 and, as it is so, the frgnt 

 end of the subocular bar 

 (.sm) rotates outwards and 

 backwards, and soon forms 

 a very considerable angle 

 with the trabeculse. The 

 Meckelian cartilage {mk) 

 at its free end becomes at 

 the same time considerably 

 elongated. These processes 

 of growth continue till (fig. 

 330) the palato-pterygoid 

 bar {Pt) forms a subocular 

 bar, and is considerably 

 longer than the original 

 subocular region of tiie 

 quadrate ; while the Meck- 

 elian cartilage {Mck) has 



