42 FISHES OF ILLINOIS 



Order ISOSPONDYLI 



(HERRING-, SHAD-, AND SALMON-LIKE FISHES) 



Skeleton bony; anterior vertebrae simple, without Weberian ossicles; 

 dorsal and anal fins without spines; ventrals abdominal; an adipose fin 

 present in some families; pectoral arch suspended from the skull; meso- 

 coracoid arch well developed, as in the Plectospondyli and the ganoids, 

 forming a bridge between the hypercoracoid and the hypocoracoid; 

 opercle well developed; maxillary distinct, forming part of the margin of 

 the upper jaw; air-bladder, if present, with an open duct; gills 4, a slit 

 behind the fourth, as normally in bony fishes. 



A large and widely distributed group, including most of the 

 marine soft-rayed fishes, excepting deep-sea forms and a limited 

 number of fresh-water species. Families numerous; 4 represented 

 in Illinois. Members of some families possess strong ganoid affini- 

 ties, a gular plate and two transverse series of arterial valves 

 occurring in albiila. Fossil remains abundant. 



Key to Families of ISOSPONDYLI found in Illinois 



a. No adipose fin; belly narrow, carinated; silvery fishes. 



b. Lateral line present Hiodontidae. 



bb. Lateral line wanting. 



c. Last rays of dorsal much elongated; mouth small, low Dorosomida. 



cc. Dorsal fin normal, its last rays not elongate (elongate in some marine 



forms); mouth large, terminal, oblique Clupeidae. 



aa. An adipose fin; belly not carinated Salmonidae. 



Family HIODONTID.E 



(the mooneyes) 



Body rather deep and much compressed, covered with silvery cycloid 

 scales; head naked; belly not serrate; lateral line developed; skeleton 

 bony; vertebrae about 60, the anterior not modified; ventral tins abdomi- 

 nal; dorsal fin rather posterior; no adipose tin; caudal forked; meso- 

 coracoid present; gill-membranes free from isthmus; branchiostegals 8 to 

 10; pseudobranchiae obsolete; gill-rakers few. short, and thick; adipose 

 eyelid little developed; mouth terminal, oblique; premaxillary not pro- 

 tractile; maxillary small, articulated to end of premaxillary and form- 



