Isthmus Barriers Separating Fish Faunas 267 



Mediterranean types * crosses the Isthmus of Suez, and the dis- 

 tinctive Red Sea and Indian typesf are equally wanting in 

 the Mediterranean. The only genera which could have crossed 

 the Isthmus are certain shallow-water or brackish-water forms, 

 sting-rays, torpedoes, sardines, eels, and mullets, widely dif- 

 fused through the East Indies and found also in the Mediter- 

 ranean. The former channel, if one ever existed, had, therefore, 

 much the same value in distribution of species as the present 

 Suez Canal. 



Geological Evidence of Submergence of the Isthmus of Suez. 

 Yet, from geological data, there is strong evidence that the 

 Isthmus of Suez was submerged in relatively recent times. The 

 recognized geological maps of the Isthmus show that a broad 

 area of post-Pliocene or Pliocene deposits constitutes the Isth- 

 mus and separates the nummulitic hills of Suez from their fel- 

 lows about thirty miles to the eastward. The northern part 

 of the Isthmus is alluvium from the Nile, and its western part 

 is covered with drifting sands. The Red Sea once extended 

 farther north than now and the Mediterranean farther to the 

 southeast. Assuming the maps to be correct, the Isthmus 

 must have been open water in the late Pliocene or post- Pliocene 

 times. 



Admitting this as a fact, the difference in the fish fauna would 

 seem to show that the waters over the submerged area were so 

 shallow that the rock-loving forms did not and could not cross 

 it. Moreover, the region was very likely overspread with silt- 

 bearing fresh waters from the Nile. To such fishes as Chcctodon, 

 Holocenirus, Thalassonia of the Red Sea, or to Crenilabrus, 

 Boops, and Zeus of the Mediterranean, such waters would form 

 a barrier as effective as the sand-dunes of to-day. 



Conclusions as to the Isthmus of Suez. We are led, there- 

 fore, to these conclusions: 



1. There is no evidence derivable from the fishes of the 

 recent submergence of the Isthmus of Suez. 



2. If the Isthmus was submerged in Pliocene or post-Plio- 

 cene times, the resultant channel was shallow and muddy, so 



* As Crenilabrus, Labrus, Symphodus, Pagellus, Spondyliosoma, Sparisoma. 

 t As Ch&todon, Lethrinus, Monotaxis, Glyphisodon, etc. 



