CONTENTS. 



LECTURE IV. 



The Skull of Fishes. Cranium not distinct from spinal Column in Lancelet. De- 

 velopment of Skull in Fishes, p. 71. Permanent Arrests of its Stages exemplified 

 in the Dermopteri, p. 72. ; in the Plagiostomes, p. 73. ; and Lepidosiren, p. 78, 

 which is the Key to the Complexities of the Skull of Osseous Fishes. Piscine 

 Characters of Skeleton of Lepidosiren, p. 83. 



LECTURE V. 



Skull of osseous Fishes. Its general Form, p. 84, and manifold Functions, p. 85. ; 

 its Cavities, p. 85. ; its Ridges and Depressions for muscular Attachments, p. 85. 

 Classification of its Bones, p. 86. ; Arrangement of those of the Endo-skeleton in 

 vertebral Segments, p. 87. The Segments defined, p. 88. Primary Segments of 

 Brain, which govern the vertebral Segments of Skull, p. 89. Neural Arches, 

 p. 89. Sense-capsules, p. 101. Htemal Arches, and their Appendages, p. 104. 

 Palato -maxillary Arch, p. 105. Tympano-mandlbular Arch, p. 110. Hyoidean 

 Arch, p. 114. The splanchnic branchial Arches, p. 116. Scapular Arch, p. 

 117. Modifications of the pectoral Fins, p. 120.; their special Homology 

 with Wings, Fore-limbs, and Arms, p. 124. ; their general Homology, p. 125. 

 Structure and Homologies of the ventral Fins, p. 126. Ichthyological Abbrevi- 

 ations and Formula of the Fin- rays explained, p. 126. Linnaan Characters, from 

 ventral Fins, of the 'abdominal,' 'thoracic,' 'jugular,' and 'apodal' Orders, p. 127. 

 Fins of Plagiostomes, p. 128. 



LECTURE VL 



Dermal cranial Bones elucidated by the Skull of the Sturgeon, p. 130. ; and 

 Lepidosiren, p. 134. Relations of dermal Bones to mucous Ducts, p. 136. 

 Homologies of the opercular Bones, p. 137. Dermal Bones of the Trunk, p. 141. 

 ' Lateral Line,' what, p. 141. Structure, Homology, and Development of Scales 

 of Fishes, p. 141. ; their kinds defined, p. 142. Fishes with cycloid and ctenoid 

 Scales comparatively modern, p. 142. High Antiquity of Ganoids and Placoids, 

 p. 143. Embryonic Characters of primseval Fishes, p. 144. Development of 

 Fins, p. 144. ; permanent Arrests of its Stages in extinct Fishes, p. 145. 

 Teleology of the Skeleton of Fishes, p. 145. Adaptation of the gristly Skeleton 

 of the Shark, p. 147, and Sturgeon, p. 148, to their respective Habits. Final 

 Purpose of the large Head of Fishes, p. 149. Continued Growth of Cranium, 

 adjusted to restricted Growth of Brain, by modifications of arachnoid Tissue, 

 p. 150. Conditions of the Size, Mobility, and Complexity of tlie inferior Arches 

 of the Skull, p. 151. Advantages of the Absence of a Sacrum, and of the 

 restricted Development of the Homologues of Arms and Legs, p. 153. Ex- 

 periments showing the Uses of the different Fins, p. 156. Synonyms of the 

 Bones of the Head of Fishes, according to their special Homologies, p. 158. 

 Synonvms of the Bones of the Head of Fishes, according to their general 

 Homologies, )>. IGl. 



