LAMPREYS. 313 



(The Lampreys.) 



Skeleton cartilaginous, without ribs, limbs, shoulder 

 girdle nor pelvic elements; skull imperfectly developed, 

 without true jaws; a single median nostril; gills in the 

 form of fixed sacs, without branchial arches; gill sacs 

 typically seven on each side ; mouth nearly circular, suc 

 torial; no scales; body elongated, eel-shaped; alimentary 

 canal nearly straight and simple; no arterial bulb. 



(Class VI. ELASMOBRANCHII, the Selachians, repre 

 sented on our Atlantic Coast by many species of Sharks 

 and Skates, is here omitted, as its members are exclusively 

 marine, and it does not therefore come within the scope 

 of this treatise. Class VIII. LEPTOCAKDII, the Lan- 

 celets, is also omitted for the same reason. The latter 

 class contains, as far as now known, but a single genus, 

 BKANCHIOSTOMA or Amphioxus, with two or three species. 

 One of these, Branchiostoma caribceum, Sundevall, occurs 

 along the coast of our South Atlantic States.) 



ORDERS OF MARSIPOBRANCHII. 



* Nasal duct a blind sac, not penetrating the palate. 



HYPEROARTTA, GG-. 



** Nasal duct penetrating the palate. HYPEROTRETA, page 313. 

 G6. FAMILIES OF HYPEROARTIA. 



* Branchial sacs seven on each side intestine with spiral valve. 



PETROMYZONTID^E, 116. 



FAMILIES OF HYPEROTRETA. 



* One external aperture on each side of body, leading by six ducts 



to as many branchial sacs ; no spiral valve ; marine para 

 sites, burrowing into the bodies of other fishes. 



Myxinida, the Hag Fishes. 

 14 



