CONTRIBUTIONS TO NORTH AMERICAN ICHTnYGLOGY IV. 



t Skull without membrane bones (a ' ' ruflimontal opercular boue " in Chimai'a ) ; 



gills not free, tlic branchial openings slif-like, usually several in number; 



esoskeleton placoid, sometimes obsolete; eggs few and large. 



Elasmobranxhii, :?. 

 XX Skull with membrane bones ; gills freo ; branchial openings a single slit on 



each side, sometimes confluent ; exoskcleton various, n<5t iilacoid ; eggs 



comparatively small and numerous Pisces, 4. 



Class I.-LEPTOCARDII. 



{The Lancelets.) 



Skeleton membrauo-cartilaginous ; skull imdevelox>ed, with tlie noto- 

 cliord persistent and extending to the anterior end of the head. Brain 

 not differentiated. No heart ; the function of the heart being i^erformed 

 by pulsating sinuses. Blood colorless. Eespiratory cavity confluent with 

 the cavity of the abdomen 5 branchial clefts in great number, the water 

 being expelled through an abdominal pore in front of the vent. Jaws 

 none; the mouth a longitudinal fissure, with cirri on each side. 



Small marine animals highly interesting to the zoologist as exhibiting 

 the lowest degree of development of the vertebrate type. The class 

 inclndes but the single order Cirrofifomi. (Ae-ro?, thin ; y.ap8ia^ heart.) 

 (Subclass Leptoeardii Giinther, viii, 513-514.) 



Order A.-CIRROSTOMI. 



[The Cirrostomcs.) 



This order is equivalent to the family BrancMostomatidce. (Latin, 

 cirrus, a lock of hair; cro/aa, mouth : the mouth being surrounded by a 

 fringe of cirri.) {Cirrostomi Giinther, viii, 513-514.) 



Family I.— BRANCHIOSTOMATID^. 



{The Lancelets.) 



Body elongate, compressed, naked, colorless, with no fins, except a 

 rayless fold extending along the back, around the tail, past the vent, to 

 the abdominal jiore. Mouth inferior, appearing as a longitudinal fis- 

 sure, surrounded by conspicuous, rather stiff cirri. Eye rudimentary. 

 Liver reduced to a blind sac of the simple intestine. 



Genera two, BrancMostoma and Epigonopterus, differing chiefly in 



