54 



PE TROM YZONTIA 



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). Mordacia, Gray, and Geotria, Gray ; 

 S. America and Tasmania. 



B. 



Ammocoete larva of Petromyzon flttvtotilis, L. A, transverse section of skin, inucli enlarged. 

 B, portion of a transverse section of the notochord, enlarged. C, transverse section (of 

 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, elastica externa ; ep, 

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

 thyroid ; gl.c, glandular cell ; m.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 



