376 TELLINID^E. 



earlier stages of growth and abruptly becoming white after- 

 wards : epidermis minutely wrinkled, fibrous at the sides and 

 edges of the shell, light brown, or occasionally greenish, mostly 

 effaced in full-grown specimens : margins rounded in front and 

 on the anterior side, somewhat produced on the posterior side 

 and ending in an obtuse angle, with a greater or less degree 

 of flexure, and frequently with an angular ridge running from 

 the posterior end to the beaks ; dorsal margin sloping with a 

 slight curve: beaks small, inflected, nearly contiguous; um- 

 bones scarcely projecting: lunule lanceolate, not very distinct : 

 ligament strong, prominent, dark horncolour, now and then ex- 

 tending between the beaks to the lunule : hinge-line curved : 

 hinge-plate^hick and broad, occupying between a third and a 

 fourth of the circumference : teeth fixed in a shallow cavity or 

 depression in the middle of the hinge-plate, and set either 

 across it or in opposite directions ; they consist of two small 

 symmetrical cardinals in each valve, the posterior in the right 

 valve and anterior in the left being bifid or cloven length- 

 wise : inside glossy and nacreous, often partaking of the ex- 

 ternal colour, and microscopically wrinkled : pallial scar large, 

 triangular, extending in a line parallel with the front margin : 

 muscular scars deep and distinct. L. O7. B. 0*8. 



Var. 1. attenuata. Shell smaller, more compressed, and of a 

 thinner consistency, often eroded ; epidermis also thin, partially 

 iridescent ; teeth very slight ; inside chalky-white. 



Var. 2. minor. Shell smaller and more triangular. L. 0-5. 

 B. 0-55. 



Var. 3. nivea. Shell of the same size as the last variety, 

 but more compressed, snow-white. 



HABITAT : Sand, gravel, and mud between tide-marks 

 on every shore. Var. 1. Southampton (J. G. J.); Or- 

 well estuary (Dr. Clarke). This is the Baltic form. 

 Var. 2. Bantry Bay (Humphreys and J. G. J.) Var. 3. 

 Llanrhidian Marsh in the estuary of the Burry River, 

 Glamorganshire (J. G. J.) Our usual form (which may 

 be termed solidula) abounds in all the later tertiary de- 

 posits, including the boulder-clay or ' till' and the Mam- 

 malian Crag. It may therefore be regarded in the main 

 as a northern species; but it is likewise common in 



