POMOXIS — CRAPPIES 237 



Gexus POMOXIS Rafinesque 



(CRAPPIES) 



Bodv moderately elongate, deep and strongly compressed; opercle 

 emarginate behind; preopercle and preorbital finely serrated; mouth 

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

 tines, entopterygoids, 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 sparoides 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' com- 

 pany by their relations to their environment instead of being sepa- 

 rated as competitors, is shown by a comparison of the coefficients of 

 association of the two crappies, on the one hand, and of one of these 

 crappies and the common bluegill (Lcpomis pallidus) on the other. 

 With 167 available collections of Pomoxis annularis and 178 of 

 sparoides, we find 66 joint occurrences, giving us a frequency of 

 association of 2 . 53 Comparing, < >n the other hand, Pomoxis annu- 

 laris 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 association of 2 . 13. 

 The larger number of collections of the two unlike species gives us 

 a relative frequency of joint occurrence distinctly less than that i if 

 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 comparatively rare 

 round sunfish (Centrarchus macroptcrus). The mouth is also large 

 for a sunfish, its opening being considerably increased by the unusual 

 length of the lower jaw. These characters of the feeding structures 

 are represented in the food by the presence of fishes, and by the 

 quantities of Entomostraca taken in spring. 



