42 EAMBLES IN SEARCH OF SHELLS. 



striped appearance which no doubt suggested the 

 trivial name, " zebra " mussel (PL V., fig. 4). It 

 is said to be met with occasionally even in unnavi- 

 gable waters (cf. Strickland, Mag. Nat. Hist., N.S., 

 1838, ii. p. 362; Bell, Zoologist, 1843, p. 253; 

 and Wolley, Zoologist, 1846, p. 1420). 



Passing now to some of the smaller bivalves, the 

 collector will have no difficulty in procuring in and 

 around London two or three species of the genus 

 Sphterium, or, as it used to be called, Cyclas. Let 

 the reader imagine little mussels somewhat spherical 

 or lens-shaped, and about the size of a sixpence or 

 less, and he will have some idea of their appearance. 

 They are found in canals, slow rivers, ponds, and 

 even ditches, and are very generally distributed. 

 The commonest kind, perhaps, is corneum (PI. V., 

 fig. 1), of which there are several varieties, and one 

 of which, pisidioides, was first described by Dr. 

 Gray (Ann. Mag. Nat. Hist., xviii. p. 25) from spe- 

 cimens found in the Paddington Canal. The shell 

 is nearly globular and equilateral, thin, glossy, and 

 of a yellowish horn colour, often with paler bands or 

 zones, denoting the periods of growth. S. rivicola 

 (PI. Y., fig. 3) is much larger, and oval instead of 



