394 Mr. A. H. Cooke on the 



Sea, but on insufficient grounds.) It seems therefore likely, on 

 a priori grounds alone, that representatives of the remaining 

 four species exist also in the Red Sea in the typical form, and 

 not, as Issel holds, as varieties. 



My own views, as will have been gathered from preceding 

 papers, while entirely discarding the extravagancies of Philippi, 

 recognize a much closer connexion between the Mediterranean 

 and Red-Sea species than does Issel. The similarity or dis- 

 similarity, the union or separation of species cannot be settled 

 in an offhand manner by the brief examination of a few picked 

 museum specimens, but must be the result of a patient com- 

 parison of large numbers of examples iii every stage of groivth 

 and in different phases of modification. Forms at first sight 

 distinct will often be united by the discovery of an interme- 

 diate form, combining or modifying the peculiarities of both ; 

 and the more the investigation of the sea-bottom is carried 

 on, the more these intermediate forms will inevitably occur. In 

 the old days, when the conchologist was the collector and 

 nothing else, an intermediate form was to him a hete noire — 

 a creature which ran foul of his monographs and threatened 

 to diminish the number of his species, and accordingly had to 

 suffer suppression or destruction. The science has taken a 

 turn since then, or rather has begun to deserve the name, and 

 an intermediate form is now welcomed as an explanation, not 

 scouted as a puzzle. 



After the most careful examination of large series, drawn 

 from both seaSj I have come to the conclusion that the follow- 

 ing species are, at the present time, common to the Mediter- 

 ranean and the Red Sea ; that is to say, that between specimens 

 taken from the two seas no point of permanent varietal differ- 

 ence, however small, can be named which is not disproved by 

 the examination of a large number of specimens. I believe, 

 too, that if such specimens were mixed up together, a thoroughly 

 good conchologist would be unable to separate them : — 



Cerithium (Pirenella)mamniillatum, Modiolaria marmorata, Forb. 



lilsso ( = Caillaiidi, Pot. Sf Mich.). ( = coenobita, VailL). 



Emarginula elongata, Costa. Area lactea, L. 



Chiton (Lophyrus) siculus, Gray Veiierupis irus, i. ( = macrophylla, 



(=:affirii.s, Issel). Desh., + derelicta, Desk. &c.). 



. (Acantbochites) discrepaus, Petricola lithophaga, i^efe. ( = Hem- 



Brown. pricbii, Issel). 



Yolvula acuminata, Brug. Tellina balaustiua, Poli (=Isseli, 



Philiue aperta, L. ( = Vaillanti, //. Ad.). 



Issel). Gastrocba^na dubia, Peim. (=Iliip- 



(Leuconia denticulata, Mont.) pellii, Desh.). 



Pecten varius, i. PPbolas dactylus, L. (=erythr0ea, 



Lima inflata, Chemn. Gray). 

 Spoudylus gsederopus (= species 



liuown as aculeatus, Cbemu.). 



