INTERMEDIATE FORMS 



CHAP. 



Fig. 14. — Examples of the Auriculi- 

 dae : A, Auricula Judae Lam., Bor- 

 neo ; B, Scarabus Lessoni Blainv., 

 E. Indies ; C, Cassidula mustelina 

 Desh,, N. Zealand ; D, Melampus 

 castaneus Miihlf., S. Pacific ; E, 

 Pedipes quadridens Pfr., Jamaica. 



have yet never become, in respect of habitat, genuine fresh- 

 water species. Like Potamides, they haunt salt marshes, man- 

 grove swamps, and the region 

 about high-water mark. In some 

 cases ( Otina^ Melampus^ Pedipes) 

 they live on rocks which are 

 moistened, or even bathed by the 

 spray, in others (Cassidula^ Auri- 

 cula) they are immersed in some 

 depth of brackish water at high 

 tide, in others again (^Scarabus) 

 they are more definitely terres- 

 trial, and live under dead leaves 

 in woods at some little distance 

 from water. Indeed one genus of 

 diminutive size (Carychium) has 

 completely abandoned the neigh- 

 bourhood of the sea, and inhabits 

 swampy ground almost all over 

 the world. 



To this same section Gehydrophila have been assigned two 

 remarkable forms of air-breathing "limpet," Siphonaria and 

 G-adinia (see page 151), and the aberrant Amphibola, a unique 

 instance of a true operculated pulmonate. Sipho- 

 naria possesses a pulmonary cavity as well as 

 a gill, while G-adinia and Amphibola are ex- 

 clusively air-breathing. Siphonaria lives on 

 rocks at or above high-water mark, Gadinia 

 between tide marks, Amphibola (Fig. 15) 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 pro- 

 cess is so far complete that they have ex- 

 changed gills for a pulmonary cavity, while in Siphonaria 

 we have an intermediate stage in which both organs 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 



Fig. 15. — An exam- 

 ple of Amphibola 

 (avellana Cliem.), 

 the only true Pul- 

 monate which pos- 

 sesses an opercu- 

 lum. 



