roMOXis — CRAPPIES 337 



Genus POMOXIS Eafinesque 



crappies 



Body moderately elongate, deep and strongly compressed; operele 

 emarginate behind; preopercle and preorbital finely serrated; mouth large; 

 maxillary with a large supplemental bone; teeth on vomer, palatines, en- 

 topterygoids, and tongue; lower pharyngeals narrow, with sharp teeth; gill- 

 rakers long and slender, numerous; dorsal spines 6 to 8; anal spines 6; caudal 

 emarginate; scales feebly ctenoid. 



Eastern United States and Canada; two species, which are 

 very similar in habit, ecological relationship, and food, scarcely 

 avoiding competition, on the whole, in any way clearly discernible 

 in our data. A tendency to geographical separation is shown 

 by the fact that annularis is the more abundant southward in 

 the general area of the genus, and spar aides northward, — the 

 latter, indeed, also ranging somewhat the farther to the north. 

 That these two species are similarly related ecologically, and 

 thus drawn into each others' company by their relations to their 

 environment instead of being separated as competitors, is shown 

 by a comparison of the coefficients of association of the two crap- 

 pies, on the one hand, and of one of these crappies and the com- 

 mon bluegill {Leyomis pallidus) on the other. With 167 avail- 

 able collections of Pomoxis annularis and 178 of spar aides, we 

 find 66 joint occurrences, giving us a frequency of association 

 of 2.53. Comparing, on the other hand, Pomoxis annularis 

 and its 167 collections with the widely and similarly distributed 

 bluegill, taken 220 times, we find them taken together in the 

 same collections 56 times, equivalent to a coefficient of associa- 

 tion of 2.13. The larger number of collections of the two unlike 

 species gives us a relative frequency of joint occurrence dis- 

 tinctly less than that of the smaller numbers of collections of 

 the closely similar crappies. 



The species of this genus diverge from the other sunfishes in 

 respect especially to their numerous, long, and finely-toothed 

 gill-rakers, which make the most effective straining apparatus 

 to be found among the sunfishes, excepting only the compara- 

 tively rare round sunfish (Centrarchus macropterus) . The mouth 

 is also large for a sunfish, its opening being considerably in- 

 creased by the unusual length of the lower jaw. These charac- 

 ters of the feeding structures are represented in the food by the 

 presence of fishes, and by the quantities of Entomostraca taken 

 in spring. 



