236 BRITISH MOLLUSKS. 



New Zealand, which is almost identical, but smaller, with the 

 European C. rivicola. In the Western Hemisphere the Cyclads 

 are more numerous, with, however, very little variation of type. 

 There are five well-established species in the Northern United 

 States, and four in the Southern. Mr. Temple Prime has described 

 several more. Three to four species inhabit Jamaica, and Mr. 

 Cuming possesses one Cyclad from Tobasco, Mexico, and another 

 (C. maculata, Morelet) collected by himself in Panama. 



The animal of Cyclas, as already noticed, has the siphonic tubes 

 separated after a Httle distance, the branchial tube being the 

 longer, and is active in its habits, making vigorous use of its foot, 

 progressing by sudden jerks, performing a rotary motion in the 

 water by forced ejectments of it, floating by its foot in contact with 

 the under surface of the water, and even suspending itself by a 

 few byssus-like threads. The animal is also able to sustain life for 

 some time out of water. Living specimens of C. maculata were 

 collected by Mr. Cuming while searching among the roots of trees 

 for land-shells in the garden of a ruined convent in Old Panama, 

 where they had been left after the rainy season, and would doubt- 

 less be preserved until its recm-rence. 



The shell of Cyclas is a thin bluish- white substance, covered by 

 a firm greenish-olive or fulvous horny epidermis. The hinge is 

 composed of rather conspicuous elevated lamellar lateral teeth and 

 sublaniellar cardinal teeth, the cardinal teeth being more developed 

 in Cyclas than in Pisidium. The ligament is small and Linear, 

 sometimes scarcely apparent on the outside. 



The British species of Cyclas are : — 



1. rivicola. Shell large, oval-globose, slightly lunuled, nearly 



equilateral, densely ridge-striated, ligament apparent. 



2. cornea. Shell small, suborbicular, nearly equilateral, almost 



smooth, ligament scarcely apparent. 



3. Pisidioides. Shell moderate, oblong-oval, posteriorly broadly 



subtriangularly sloped, concentrically finely ridged. 



4. ovalis. Shell moderate, oblong-oval, compressed, nearly equi- 



lateral, smooth, umboes nearly central. 



5. lacustris. Shell small, squarely orbicular, rhombic, com- 



pressly expanded at the sides, smooth, umboes calyculate. 



