54 



PE TROM \ 7 Z0NTIA 



Both marine and freshwater Lampreys are known, but no fossil 

 forms have yet been discovered. 



Family Petromyzontidae. Petromyzon, Art. ; rivers and seas of 

 Europe, Asia, and America (Fig. 16). Mordaeia, Gray, and Geotria, Gray ; 

 S. America and Tasmania. 



Ammocoete larva of Petromyzon fluviatilis, L. A, transverse section of skin, much enlarged. 

 B, portion of a transverse s'ection of the notochord, enlarged. C, transverse section lof 

 the thyroid gland, enlarged, b, branchial basket ; c, striated club-cells ; eg, Lumen of thyroid 

 gland, which communicates with the pharynx ; c.t, connective tissue ; el, elastics externa ; ep, 

 outer layer of epidermis ; f.s, fibrous sheath; g.c, gland-cell; gd, row of glandular cells of 

 thyroid ; gl.c, glandular cell ; in.j, median jugular vein ; nt, notochord ; o, opening of thyroid 

 gland ; v.a, ventral aorta. 



Affinities. — The Cyclostomes, united to the cartilaginous fish 

 by Cuvier, were separated from the true fish by Agassiz (1857). 

 That they should be definitely placed apart from the Gnathostomes 

 the facts mentioned above leave no doubt. Such special characters 

 as the rasping ' tongue,' the large sub-cerebral hypophysial sac, 

 etc., prove that they form a single divergent group ; on the other 



