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



To this same section Gehydrophila (though separated off by some 

 authors as Thalassophila) may be assigned two remarkable forms of 

 air-breathing " limpet," Siphonaria and Gadinia, and the aberrant 

 Amphibola, a unique instance of a true operculated pulmonate. 

 Siphonaria possesses both a lung and a gill, while Gadinia and 

 Amphibola are exclusively air-breathing. Siphonaria lives on rocks 

 at or above high-water mark, Gadinia between tide marks, Amphibola 

 in brackish water at the estuaries of rivers, half buried in the sand. 

 There can be little doubt that all these are marine forms which are 

 gradually becoming accustomed to a terrestrial existence. In 

 Gadinia and Amphibola the process is so far complete that they have 

 exchanged gills for lungs, while in Siphonaria we have an intermediate 

 stage in which both gill and lung exist together. A curious 

 parallel to this is found in the case of Ampullaria, which is furnished 

 with two gills and a pulmonary chamber, and breathes indifferently 

 air and water. It is a little remarkable that Siphonaria, which lives 

 at a higher tide level than Gadinia, should retain the gill, while 

 Gadinia has lost it. 



The ultimate affinities of the essentially freshwater groups, 

 Limmca, Physa, Chilina, cannot be precisely affirmed. The form 

 of shell in Latia, Gundlachia, and perhaps Ancylus, may suggest to 

 some a connection with the Otinidai, and in Chilina, a similar 

 connection with the Auriculidce. But, in a question of derivation, 

 similarities of shell alone are of little value. It is not a little remark- 

 able, for instance, that we should find a simple patelliform shell in 

 genera so completely distinct from one another in all anatomical 

 essentials as Ancylus, Patella, Siphonaria, Propilidium, Hippotiyx, 

 Cocculina, and Umbrella. 



Bouvier, on grounds of general organisation, regards* the 

 Hygrophila in general as Opisthobranchs adapted to an aerial life. He 

 considers that the Nudibranchiate Opisthobranchs have given birth to 

 the Pulmonaia stylom?natophora, and the Tectibranchiate Opisthobranchs 

 to the Pulmonata basommatophora. Such a view is much more easily 

 stated than definitely disproved, but it seems open to serious 

 objection from other views than those which deal simply with 

 anatomy. The Opisthobranchiata are not, to any marked extent, 

 littoral genera, nor do they specially haunt the mouths of rivers. On 

 the contrary, they inhabit, as a rule, only the very lowest part of the 

 littoral zone, and are seldom found except where the water is purely 

 salt. In other cases, when the derivation of land or freshwater 

 genera is fairly well established, intermediate forms persist, which 

 indicate, with more or less clearness, the lines along which 



* Le Naturalists, 1889, p. 242 f. 



