xii PREFACE. 



been derived from the gradual shifting, atrophy, hypertrophy, and con- 

 crescence of primitively similar cartilaginous rods, which formed a series on 

 each side of the body, identical in character with the primitive median 

 dorsal series. According to this view, the " archipterygium " of Professor 

 Gegenbaur is not antecedent to, but is derived from the type of fin found 

 in Elasmobranchs. (See also on this subject, Huxley, On Ceratodus, Proc. 

 Zool. Soc. vol. 1876, p. 24.) 



Eelation of the Malleus and Incus to the Mandibular and 

 Hyoid Arches. — Investigations directed to the development of the skull 

 led Professor Huxley some years since to adopt the conclusion of Eeichert 

 and of Goodsir, that the small bones of the Mammals' tympanic cavity were 

 derived from the upper ends of the anterior visceral arches. At first it 

 appeared probable that the malleus and incus were both derived from the 

 upper end of the cartilaginous mandibular arch, the lower part forming 

 Meckel's cartilage. This led to the suggestion that the malleus corresponds 

 to the articulare of the lower jaAv of other Vertebrata, whilst the incus was 

 considered to be the representative of the quadratum, since it articulates 

 with the malleus just as the quadratum does with the articulare (Croonian 

 Lecture " On the Theory of the Vertebrate Skull," Proc. Loyal Society, 

 vol. ix. p. 398). 



Further investigation led Professor Huxley to a modification of his 

 views. The embryological evidence is not quite complete, but the relations 

 of the parts in question in the developing Frog, in certain Lizards, and in Mam- 

 malia, have led him to the conclusion (" Manual of Vertebrate Anatomy," 

 p. 85, 1871) that whilst the malleus is formed from the uppermost extremity 

 of the mandibular arch, and therefore represents, not articulare, but quad- 

 ratum, the incus is developed from the uppermost extremity of the second or 

 hyoid arch, and corresponds to the hyomandihular of fishes. The stapes is 

 also developed from the upper portion of the hyoid arch, just below the 

 incus. The incus may therefore be spoken of as the supra-stapedial portion 

 of the hyoid arch, and in certain Vertebrata it exists as a mere cartilaginous 

 supra-stapedial rudiment. 



These vieAvs in their later form have not been adopted by Professor 

 Gegexbaur. He observes (§ 402) that the homologies of the ossicula 

 auditiis of the various classes of Vertebrata have not yet been satisfactorily 

 determined. In § 352 he maintains the earlier determination of the homo- 

 logy of the mammalian malleus with the articulare of other Vertebrates. 

 Concerning the homologies of the incus and the stapes, he considers it 

 advisable, in the present state of knowledge, to make no statement. 



The student is advised of these differences of interpretation of structural 

 fact, in order that he may the more carefully make himself acquainted from 

 original sources with the details of development, relation to nerves, and other 

 features of the parts under discussion. 



Nomenclature of the Lobes of the Brain in Fishes. — In the 

 earlier editions of the present work, Professor Gegexbaur, led by the result 



