The Sharks.--Characters of Elasmobranchs.--Classification of Elasmobranchs.--Subclasses of Elasmobranchs.--The Selachii.--Hasse's Classification of Elasmobranchs.--Other Classifications of Elasmobranchs.--Primitive Sharks.--Order Pleuropterygii.--Order Acanthodii.--Dean on Acanthodii.--Order Ichthyotomi. 506 CHAPTER XXX. THE TRUE SHARKS. Order Notidani.--Family Hexanchidæ.--Family Chlamydoselachidæ.--Order Asterospondyli.--Suborder Cestraciontes.--Family Heterodontidæ.--Edestus and its Allies.--Onchus.--Family Cochliodontidæ.--Suborder Galei.--Family Scyliorhinidæ.--The Lamnoid, or Mackerel-sharks.--Family Mitsukurinidæ, the Goblin-sharks.--Family Alopiidæ, or Thresher-sharks.--Family Pseudotriakidæ.--Family Lamnidæ.--Man-eating Sharks.--Family Cetorhinidæ, or Basking Sharks.--Family Rhineodontidæ.--The Carcharioid Sharks, or Requins.--Family Sphyrnidæ, or Hammer-head Sharks.--The Order of Tectospondyli.--Suborder Cyclospondyli.--Family Squalidæ.--Family Dalatiidæ.--Family Echinorhinidæ.--Suborder Rhinæ.--Family Pristiophoridæ, or Saw-sharks.--Suborder Batoidei, or Rays.--Pristididæ, or Sawfishes.--Rhinobatidæ, or Guitar-fishes.--Rajidæ, or Skates.--Narcobatidæ, or Torpedoes.--Petalodontidæ.--Dasyatidæ, or Sting-rays.--Myliobatidæ.--Family Psammodontidæ.--Family Mobulidæ. 523 CHAPTER XXXI. THE HOLOCEPHALI, OR CHIMÆRAS. The Chimæras.--Relationship of Chimæras.--Family Chimæridæ.--Rhinochimæridæ.--Extinct Chimæroids.--Ichthyodorulites. 561 CHAPTER XXXII. THE CLASS OSTRACOPHORI. Ostracophores.--Nature of Ostracophores.--Orders of Ostracophores.--Order Heterostraci.--Order Osteostraci.--Order Antiarcha.--Order Anaspida. 568 CHAPTER XXXIII. ARTHRODIRES. The Arthrodires.--Occurrence of Arthrodires.--Arthrognathi.--Anarthrodira.--Stegothalami.-- Arthrodira.--Temnothoraci.--Arthrothoraci.--Relations of Arthrodires.--Suborder Cycliæ.--Palæospondylus.--Gill on Palæospondylus.--Views as to the Relationships of Palæospondylus: Huxley, Traquair, 1890. Traquair, 1893. Traquair, 1897. Smith Woodward, 1892. Dawson, 1893. Gill, 1896. Dean, 1896. Dean, 1898. Parker & Haswell, 1897. Gegenbaur, 1898.--Relationships of Palæospondylus 581 CHAPTER XXXIV. THE CROSSOPTERYGII. Class Teleostomi.--Subclass Crossopterygii.--Order of Amphibians.--The Fins of Crossopterygians.--Orders of Crossopterygians.--Haplistia.--Rhipidistia.--Megalichthyidæ.--Order Actinistia.--Order Cladistia.--The Polypteridæ 598 CHAPTER XXXV. SUBCLASS DIPNEUSTI, OR LUNGFISHES.