PERC0PSID.« THE TROUT-PERCHES 225 



Family PERCOPSIDjE 



(the trout-perchesJ) 



Body moderately elongate, somewhat compressed; caudal peduncle 

 rather long and slender; scales with edges strongly ctenoid; head naked; 

 lateral line developed; skeleton bony; anterior vertebras simple; ventral 

 fins abdominal, somewhat anterior; dorsal fin with 2 spines; ventrals 

 with 1 rudimentary spine and about 8 rays; anal with 1 or 2 spines; 

 caudal forked; an adipose fin present; no mesocoracoid ; gill-membranes 

 separate, free from isthmus; branchiostegals '6 ; pseudobranchiae pres- 

 ent; gill-rakers short, tubercle-like; opercle with entire edges; mouth 

 small, horizontal; premaxillaries not protractile; teeth very small, villi- 

 form, on premaxillaries and lower jaw only; stomach siphonal, with 

 about 10 well-developed pyloric caeca; air-bladder present, with an open 

 duct (Boulenger) ; ova large, not falling into the abdominal cavity before 

 extrusion. 



Small fishes of the fresh waters of North America ; 2 genera 

 known, each containing a single species ; one species found in Illinois. 



This family "shows the remarkable combination of true fin- 

 spines, ctenoid scales, and a percoid mouth, with the adipose fin, 

 abdominal ventrals, and naked head of the Isospondyii" (herring- 

 Jike forms). It is doubtless a surviving remnant of a fauna which 

 marked the transition from the soft-rayed herring-like forms to the 

 later-appearing groups of acanthopterygian fishes. 



Genus PERCOPSIS AgjJssiz 



(trout-perch) 



Characters in the main as above, differing from the single other 

 known genus of the family (Columbia Eigenmann, recently, described 

 'from the Pacific slope) in the weaker dorsal spines, the more translucent 

 body, and the relative absence of serration of the preopercle. Atlantic 

 slope and Great Lake region, in clear cold waters; one species. 



PERCOPSIS GUTTATUS Agassiz 



(trout-perch) 



•Agassiz, 1850, Lake Superior, 286. 



G., VI, 207; J. &G., 322; M. V., 82; J. & E., I, 784; N., 43; J., 53; F., 72; L. 22. 



Length 6 inches; body elongate, not much compressed, strongly 

 .tapered posteriorly, the caudal peduncle slender; depth 3.9 to 4.5; 

 greatest width § greatest depth ; depth caudal p.eduncle 2 . 7 to 3 . 2 in its 



