Jordan and Evermann. — Fishes of North America. 783 



Family CIV. PERCOPSID^. 



(The Sand Rollers.) 



Body moderatf^Iy elongate, somewhat compressed, the caudal peduncle 

 long and slender. Head conical, pointed, naked. Mouth small, hori- 

 zontal ; maxillary short, narrow, without supplemental bone, not reach- 

 ing to the large eye; margin of upper jaw formed by premaxillaries 

 alone, which are short and not protractile. Teeth A^ery small, villiform 

 on premaxillaries and lower jaw only. Tongue short, adherent. Gill 

 membranes separate, free from the isthmus. Pseudobranchise present. 

 Branchiostegals 6. Gill rakers short, tubercle-like. Opercle with entire 

 edges. Lower limb of the preopercle well developed, the angle nearly a 

 right angle, its inner edge with a raised crest, its outer edge crenulate or 

 with a few spines. Bones of the head cavernous, as in the Percoid genus 

 Acerina ; cranium with a raised crest, which does not extend to the occi- 

 put. Scales moderate, rather firm, adherent, their edges strongly cten- 

 oid. Lateral line continuous. Dorsal short, median, with 2 spines, 

 slender or stout ; ventrals anterior, just in front of the dorsal, with 1 

 rudimentary spine and about 8 rays ; pectorals narrow, placed rather 

 higher than usual in IsospondijU ; anal small, with 1 or 2 spines ; caudal 

 forked ; adipose fin present, small. Vertebrae about 35. First superior 

 pharyngobranchial without teeth : second, third, and fourth separate, 

 with teeth. Lower pharyngeals separate. Stomach siphonal, with about 

 10 well-developed pyloric cceca. Ova unusually large, not falling into 

 the abdominal cavity before exclusion. Air bladder present, with a band 

 of connective tissue which is apparently with a rudimentary duct. Small 

 fishes of the fresh waters of the cooler parts of America ; two genera 

 known, each with probably but one species. The group is one of special 

 interest, as it combines with ordinary Salmonoid characters the structure 

 of the head and mouth of a Percoid, resembling notably the European 

 genus Gymnocephalns or Acerina. The late discovery by Dr. Eigenmann 

 of a second genus, still more decidedly percoid in its structure is the most 

 interesting recent addition to our knowledge of American fishes. (Pcr- 

 copsidcc, Giinther, Cat., Yi, 207, 1866.) 

 a. Dorsal fin with 2 feeble, slender spines or simple rays; anal with 1 slender spine; scales 

 most strongly ctenoid on caudal peduncle; posterior margin of preopercle entire or 

 with feeble crenulations; lateral line developed, the tubes small; form slender, the 

 body translucent. Peecopsis, 349. 



aa. Dorsal and anal each with 2 very strong spines; ventral spine evident; scales most 

 strongly ctenoid on anterior part of body; posterior margin of preopercle with a few- 

 short but strong spines; lateral lino imperfect, the tubes more or loss obsolete; form 

 robust, the substance comparatively opaque. Coi.umbia, 350. 



349. PERCOPSIS, Agassiz. 



PercopsU, Agassiz, Lake Superior, 284, 1850, (guflafiiK). 



Sahnopercn, Thompson, App. Hist. Vermont, 33, 1853, (j«7?»cii/a = ./»»<i/«s). 



Body rather slender, pellucid, covered with rather thin scales. Dorsal 

 fin with 2 slender spines or simple rays; anal with 1; scales roughest 



