MoUuscan Fauna of the Oulfof Suez. 393 



" geographical varieties " to Red-Sea species akin to Medi- 

 terranean and liis distinction between " equivalent species ^' 

 and " equivalent varieties " alike fail, and fail for the very- 

 reasons he gives for establishing them. That Mediterranean 

 species migrated into the Red Sea both in Pliocene and Post- 

 pliocene times no one can deny ; but I do not believe that an 

 examination of the recent species alone, as now existent on 

 both sides of the isthmus, will enable us even approximately 

 to conclude at Avhich of the two periods particular species 

 migrated. Issel assumes that the amount of variation from a 

 supposed Mediterranean type is a fair measure of the time at 

 which the variation began; in other words, of the time when 

 the separation took place. Nassa circumcincta varies much 

 from N. gihbosula^ therefore it came over in the Pliocene 

 period ; N. costulaia, var. erythrcea^ varies little from N. costu- 

 lata, therefore it did not come over till the Postpliocene. 

 This I should deny entirely, because it assumes as a basis of 

 comparison what does not rest upon a shadow of foundation, 

 viz. that all species form varieties with equal rapidity, and 

 that it takes a longer time for a species to form a marked 

 variety than it does to form a slight one. 



The following marine shells, now living in the Mediter- 

 ranean, occur in Postpliocene beds at Suez : — 



Gastrochsena, sp. (prob. dubia, Modiolaria coeiiobita,P'ae7/. (= mar- 

 Perm.), niorata, Forb.). 



Solecurtus strif,'ilatus, L, Nassa niutabilis, L. 



Petricolatlempricbii, isse/( =11 tho- costulata, Ren. 



pbaga, Retz), Murex trunculus, L. 



Area lactea, L. Calyptrsea cbiueusis, L. 



—— No£e, L. Patella c^erulea, L. 



Donax trunculus, L., and Cardium if>tkmicum, Issel 

 ( = edule, L., var.), occur in the raised beaches of the Bitter 

 Lakes, but not at Suez. 



Five of these species (if Area Noce is not a misidentifica- 

 tion of arahica, Forsk., and Patella coirulea of rota, Chemn.) 

 are no longer living in the Red Sea, viz. Nassa mutabilis, 

 Murex trunculus, Calyptrma chinensis, Patella coiriilea, and 

 Area Note. Why these species shovild have ceased to exist 

 in the Red Sea while others have lived and flourished is a 

 point of which no satisfactory explanation can be offered. It 

 is certainly not a case, where littoral species have succumbed to 

 a great increase of temperature. On the other hand, it is 

 noticeable that of the remaining six species, Issel allows that 

 two, viz. Petricola Hemprichii and Modiolaria coenohita, still 

 live in the Red Sea in the typical form. [Murex trunculus and 

 Nassa mutahilis have both been reported as living in the Red 



