anadromous, and catadromous types : (1) the southern lampreys, 

 comprising the genera Mordacia and Geotria, each referable to a f amity 

 distinct from the Holarctic Petromyzontidae, and the distinctive and 

 primitive teleost families Galaxiidae, Aplochitonidae, Retropinnidae, and 

 Prototroctidae. Their trenchant separation from not clearly recognizable 

 Holarctic representatives indicates prolonged isolation. 



A few other primitive teleosts, notably the subantarctic herrings 

 usually retained in the genus Chipea, may be pre-Tertiary or early 

 Pleistocene relicts, for the}^ are of ancient lineage. Their degree of 

 differentiation and their far-southern habitat suggest isolation since one 

 of the earlier, intense periods of Pleistocene glaciation. 



Bitemperate fish types not only outnumber the bipolar, but also, 

 as a rule, exhibit less differentiation and appear to have had their popu- 

 lations disrupted by tropical waters for a briefer period. The species 

 that exhibit no measured differences on the two sides of the Tropics 

 and those that are only incipiently or weakly differentiated have probably 

 been segregated into northern and southern populations only since late 

 Pleistocene. 



Many of the bitemperate types — and among these we might better 

 include some of those here listed among the bipolar — may have crossed 

 the Tropics, perhaps chiefly in Pleistocene times, by descending into 

 deeper water. This is particularly probable for various more or less 

 strictly bitemperate types of sharks, including the genera Hexanchus, 

 Heptranchias, N otorhynchus , Heteyodontus, Cephaloscylliimi, some other 

 scyliorhinids, Galeorhintis, Mitsuktmna, Carcharias, Lamna, Tetroras 

 [Cetorhimis], Somniosus, and Echinorhinits, and certain species-groups of 

 Mustelus and Squalus. Other bitemperate fish types that may have 

 transgressed the Tropics by a deep-water isothermal route during the 

 Pleistocene are the hagfish genera Eptatretus and Polistotrema ; the 

 john-dory [Zeus), an example of wide-spread Old World antitropical 

 distribution ; also, in the Eastern Hemisphere, various gurnards (Trigli- 

 dse), sinistral flatfishes (Bothidie), and other groups ; in the eastern Pacific, 

 such genera as Cheilotrema, Pinielometopon, Caulolatiliis, and Sicyogaster ; 

 the scorpaenid genus Sebastodes, a speciose North Pacific genus of which 

 one species group has populated waters from Peru to Cape of Good Hope ; 

 Oplegnathus ; and certain clinids. 



Many other antitropical fishes of the Temperate Zones, including 

 most of the pantemperate types, have attributes that lead us to beheve 

 that they crossed the Tropics when the surface waters were considerably 

 cooled, presumably during late Pleistocene time. Ecologicalty, they are 

 surface-bound ; therefore incapable of transgressing equatorial water by 

 the cool deep-water route. That they crossed the Tropics during the 

 last cold period of the Pleistocene is suggested by their incipient 

 speciation. 



In this category of surface-bound recent transgressors of the Tropics 

 we must include such shore-pelagic fishes as the sardine or pilchard genus 

 Sardinops (of . Cahfornia, Japan, Peru and Chile, Austraha and New 

 Zealand, and South Africa) ; the roundherrings of the genus Etrumeus 

 (of the western North Atlantic, Cahfornia, Hawaii, Japan, Austraha, and 

 South Africa) ; the anchovy genus EngrauUs (of Europe, Japan, South 

 Africa, and Australia and New Zealand) ; the similar anchovies of 



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