409 
of species found by us in the entire area of these two districts is con- 
sequently 73. The ichthyological affinity of these two areas is evi- 
dently to be measured by the ratio which the number of species com- 
mon to both bears to the whole number of species found in either or 
both the areas—in this case, the ratio of 26 to 73, or 36 per cent. 
That is, 36 per cent. of the fishes found in either of these two districts 
have been found by us in both of them. 
A similar analysis of the data for each of the forty-five pairs 
which it is possible to make up from our ten hydrographic districts, 
yields the material for the following table of common species and of 
ratios of affiliation. This table shows, in the lower left-hand part, 
NUMBER OF SPECIES COMMON TO EACH PAIR OF DISTRICTS, AND RATIOS 
OF SUCH COMMON NUMBERS TO THE WHOLE NUMBER 
OF SPECIES IN EACH PAIR 
5 aan a SS 
Ss j Qa, 24 KS) 
ale ae | ele le | 3 
District s pete esas ||. arc ette| SON) eae ice 
istricts 5 | 2 ae al os al ae 43) | u 
TE Se a [a Peal Pa fa Wea (Py Dea fa 
Oy || TEASER ec eli cen ola rere 
alafo}wr{[wi[ol[rj[dedlajls 
| 
Hy PRISER aias a wine <a vs A532’ | 20.) 41.| 40 | 38 | 28 | 36.) 37 | .352 
Ditty ROCK MREVOE. <t2. 06 2ce.00 42 OSh 35) 1/769 3) 59) 63: 40) | 47) °62> ||| . 542 
| 
3. Illinois River ....... 42 | 89 PSS W420 (253%), 660) 33°) 447) G8 252 
A Bake ICIS AT a. 2s Bh oy.y 5) 5. LE NSO AS Nee aioe 25: 29 le 22).| QS. 324i 2283 
Bi) MISSISSIPPls.cs.c 6 ce sa Ae ep Oda SOe Wes .| 54) OU | 34.42 1°66. |) 5525 
6. Kaskaskia ......... 2) (60) 268 | 25 |58 |x. .|6G | 52163 | 83 |) 547 
(LAE EES ee Beeler 2 ieSo | 34 M73" GG: Wis | 40 |58 | 63. |. e584 
8. Big Muddy.......... HOM eeS |) wawiede |. 35438" 40:1. .|°70.| 39%) 2398 
9. Saline River...2..... 26 | 47 | 53 | 21 | 45 | 48 | 52 | 40 |....| 49 | .471 
fOrpiCrmetees. ooh. 2 a9) ie) Oa) se | e959 176.) 40 | Say) o)..| 524 
om | 
Total species............. 44 | 92 | 128] 57 | 97 | 69 | 95 | 42 | 55 | 101] 
Number of collections... 13 | 73 |1115) 20 | 57 | 41 | 103] 10 | 18 | 95 | 
