LAMPKEYS. 347 



(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. — Elasmobraxchii, 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. — Leptocaedii, the Lan- 

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

 class contains, as far as now known, but two genera, 

 JhJ2)igo7i02yterus2indi Artiphioxus^ with two or three species. 

 One of these, Amphioxus carihceus^ Sundevall, occurs 

 along the coast of our South Atlantic States.) 



ORDERS OF MARSIPOBRANCHII. 



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



Hyperoartia, GG. 



** Nasal duct penetrating the palate. Hyperotreta, page 347 



GG. FAMILIES OF HYPEROARTIA. 



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



Petromyzontid^, 116. 



FAMILIES OF HYPEROTRETA. 



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



lo as nian}^ branchial sacs; no spiral valve; marine para- 

 sites, burrowing into the bodies of other fishes. 



Myxinidmy the Ilag Fishes. 



