42 FISHES OF ILLINOIS 



Order ISOSPONDYLI 



heering-, 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 Plecfospondyli and the ganoids, form- 

 ing a bridge between the hypercoracoid and the hypocoracoid ; opcrcle 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 repre- 

 sented in lUinois. Members of some families possess strong 

 ganoid affinities, a gular plate and two transverse series of 

 arterial valves occurring in Albula. Fossil remains abundant. 



Key to Families of ISOSPONDYLI fouxd ix Illinois 



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



b. Lateral line present H iodontidse. 



bb. Lateral line wanting. 



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



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 fins abdominal; dorsal 

 fin rather posterior; no adipose fin; caudal forked; mesocoracoid 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 protractile; maxillary small, 



