82 CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY IV. 



Subclass CHOJNDROSTEI. 



{The Sturgeons.) 



Skeleton partly cartilaginous. Ventral fin abdominal, with an entire 

 series of basilar segments. ISTo suboperculum or preoperculiim. Branchi- 

 ostegals (single or) wanting ; a prcecoracoid arch ; no symplectic bone. 

 Mesopterygium distinct ; iuterclavicles present.* Arterial bulb with 

 several pairs of valves. Optic nerves forming a chiasma. Intestine 

 with a spiral valve. Air-bladder connected by a duct with the oesopha- 

 gus. Tail heterocercal, its fin with fulcra. Skin naked or armed with 

 bony i)lates. This group comprises two orders. 



(Order Chondrostei Giinther, viii, 332-347. x^^''^P"Cy cartilage ; uariov, bone.) 



ANALYSIS OF OKDEKS OF CHONDROSTEI. 



* Maxillary and iuteropercle obsolete; skin naked; brancbihyals cartilaginous; air- 

 bladder cellular Selachostomi, G. 



** Maxillary and interopercle present ; skin -with bony sbields ; brancbibyals osseous ; 

 air-bladder simple Glaniostomi, H. 



Okder g.-selachostomi. 



{The Paddle-Jishes.) 



A prajcoracoid arch ; no symplectic bone ; premaxillary forming mouth 

 border ; no suboperculum, preoperculum, nor interoperculum j mesop- 

 terygium distinct ; basihyals and superior ceratohyal not ossified ; inter- 

 clavicles present ; maxillaries obsolete ; branchihyals cartilaginous. 

 This order contains but one family, Poli/odontidw. {frsMyy]^ shark ; aroiia^ 

 mouth.) 



Family XXV.— POLYODONTIDJE. 



{The Faddle-Jishes.) 



Body fusiform, little compressed, covered with mostly smooth skin. 

 Snout prolonged, expanded into a thin, flat blade, the inner portion 

 formed by the produced nasal bones, the outer portion with a reticulate 

 bony frame- work, the whole somewhat flexible. Mouth broad, termi- 

 nal, but overhung by the spatulate snout, its border formed by the pre- 

 maxillaries, the maxillaries being obsolete. Jaws with many fine decid- 



* The osteological characters here and elsewhere in this work are mostly taken from 

 Cope's " Contribution to the Ichthyology of the Lesser Antilles". Trans. Am. Phil. 

 Soc. 1870. 



