COOKE : ORIGIN OF THE GENERA OF L. AND F. W. MOLLUSCA. 47 



modification has proceeded. No such undoubted links can be 

 shown to exist, or to have existed, in the present case, between the 

 TecHbranchiate Opisthobranclis and the Limnaeidce. Bouvier indeed 

 finds this link in the Siphonariida and Amphibolidce, but the 

 connection is of the slightest. It might be held, with equal, or more, 

 probability, that the Hygrophila are in reality derived from the land 

 Pulmonata, a section of which may be supposed to have betaken 

 themselves to an aquatic life. Or we may hold, with Pelseneer, 

 that the reverse process has taken place, and that the Basom- 

 matophora are the direct ancestors of the Stylommataphora. In 

 this case, Succinea would be an intermediate link, with a curious 

 parallel in Onchidiwn, a pulmonate which has retrogressed* to a 

 semi-marine habit of life. 



ORIGIN OF THE LAND FAUNA. 



GASTEROPODA. (1) Operculate. 



On a priori grounds, one might predict a double origin for land 

 operculates. Marine species might be imagined to accustom them- 

 selves to a terrestrial existence, after a period, more or less prolonged, 

 of littoral probation. Or again, freshwater species, themselves 

 ultimately derived from the sea, might submit to a similar trans- 

 formation, after a preliminary or intermediate stage of life on 

 mudbanks, wet swamps, branches overhanging the water, &c. Two 

 great families in this group, and two only, seem to have undergone 

 these transformations, the Littorinida and the Neriiidce. The 

 derivation of all existing land operculates may be referred to one or 

 other of these groups. 



The power of the Littorinida. to live for days' or even weeks without 

 being moistened by the sea may be verified by the most casual 

 observer. In the tropics this power seems even greater than on our 

 own shores. I have seen, in various parts of Jamaica, Littorina 

 muricata living at the top of low cliffs among grass and herbage. 

 At Panama I have taken three large species of Littorina (varia, 

 fasciata, pulchrd) on trees at and above high water-mark. Cases 

 have been recorded, in which a number of L. muricata, collected and 

 put aside, have lived for three months, and L. irrorata, for four 

 months. f These facts are significant, when we know that the land 

 operculates almost certainly originated in a tropical climate. 



The Cyclophoridce, Cydostomatidce, and Aciculidce, which, as 

 contrasted with the other land operculates, form one group, have 

 very close relations, particularly in the length and formation of the 

 radula, with the Littorinida. 



* Bergh, Morphol. Jahrb. x. 172 f. t Calkins, Amer. Nat. xi. p. 687. 



