xni PHYLUM CHORDATA 139 



significant archaic character. The total absence of limbs may be 

 a result of degeneration. 



The geographical distribution of the class is interesting from 

 the fact that each order contains some genera which are mainly 

 northern, others which are exclusively southern. Petromyzon is 

 found on the coasts and in the rivers of Europe, North America, 

 Japan, and West Africa ; it is therefore mainly Holarctic. Ichthyo- 

 mvzon is found on the western coasts of North America, Mordacia in 

 Tasmania and Chili, Geotria in the rivers of Chili, Australia, and 

 New Zealand. Myxine occurs in the North Atlantic and on the 

 Pacific Coast of South America ; Paramyxine in the Pacific ; 

 Bdellostoma on the coasts of South Africa, New Zealand, and Chili. 



No undoubted fossil remains of Cyclostomes are known, but 

 there is some reason to believe that a little fossil, Palceospondylus 

 gunni (Fig. 820), discovered in the Devonian rocks of Scotland, 

 may be referable to this class. It is about an inch long, and shows 

 two regions, the cranium and the vertebral column ; there is no 

 trace of exoskeleton or teeth. The vertebral column is composed 

 of calcified centra with neural arches ; haemal arches are present in 

 the caudal region ; the structure of this part of the skeleton is thus 

 of a distinctly higher type than in recent Cyclostomes, and this 

 perhaps lends support to the view that the latter are degenerate. 

 There is a caudal fin supported by slender, sometimes forked, rays. 

 The cranium consists of an anterior, probably trabecular, region 

 (t.p.), and of a posterior region (p.a.), which seems to answer to 

 the parachordals and auditory capsules. Just in advance of the 

 anterior region is a ring-shaped opening surrounded by cirri (c.) : 

 this may be either the nasal aperture or the mouth. There are 

 vestiges of upper and lower jaws, and about four branchial arches. 

 The posterior region of the skull gives off paired plates (x) which 

 may perhaps represent pectoral fins. 



CLASS II.-PISCES. 



The Pisces, including the cartilaginous Fishes, the bony Fishes, 

 and the Dipnoi, are Craniata which have the organs both of respira- 

 tion and of locomotion adapted for an aquatic mode of life. The 

 chief, and in the majority the only, organs of respiration are the 

 gills, which are in the form of series of vascular processes attached 

 to the branchial arches and persisting throughout life. The organs 

 of locomotion are the paired pectoral and pelvic fins, and the 

 unpaired dorsal, ventral, and caudal ; these are all supported 

 by fin-rays of dermal origin. A dermal exoskeleton is usually 

 present. In the endoskeleton the notochord is usually more or 

 less completely replaced by vertebra ; there is a well-developed 

 skull, and a system of well-formed visceral arches, of which the first 

 forms upper and lower jaws, the latter movably articulating with 

 the skull, and both nearly always bearing teeth. There is frequently 



