XIII 



PHYLUM CHORDATA 



143 



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. 

 Ichthyomyzon 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 Cyclo- 

 stomes are known, but there is some 

 reason to believe that a little fossil, 

 Palwospondylus gunni (Fig. 810), dis- 

 covered in the Devonian rocks of Scot- 

 land, may be referable to this class. It 

 is about an inch long, and shows two re- 

 gions, the cranium and the vertebral 

 column ; there is no trace of exoskeleton 

 or teeth. The vertebral column is com- 

 posed 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. 



FIG. 810. Palaeospondylus 



gunni (magnified), c. cirri ; 

 p. a. parachordal and auditory 

 region ; t.p. trabecular re- 

 gion ; x, backward processes 

 of skull. (After Traquair.) 



