PISIDIUM. 21 



SHELL subtriangular, somewhat ventricose, thin, rather 

 glossy, finely but irregularly striate concentrically, greyish- 

 white : epidermis very thin : anterior side abruptly truncate : 

 posterior side rounded, and sloping gently towards the lower 

 margin ; the anterior and posterior margins are compressed, 

 especially towards the beaks, on each side of which they form 

 a kind of shoulder : beaks prominent and rather acute : liga- 

 ment very short and scarcely perceptible : inside white and 

 nacreous : hinge short, but very strong ; dentition as in P. 

 amnicum, except that the cardinal teeth do not assume the 

 shape of an inverted V : muscular and pallial scars the same 

 as in that species. L. 0-15. B. O175. 



Var. 1. Henslowana. Each valve furnished with a plate- 

 like appendage near the beaks. Tellina Henslowana, Shep- 

 pard in Linn. Trans, xiv. p. 150. P. ffenslowianum, F. & 

 H. ii. p. 131, pi. xxxvii. f. 11. 



Var. 2. pulchella. Shell more glossy, strongly and regu- 

 larly grooved ; beaks less acute. P. pulchellum, Jen. p. 18, 

 tab. xxi. f. 1-5 ; F. & H. ii. p. 128, pi. xxxvii. f. 12, 13. 



Yar. 3. pallida. Shell more ventricose, irregularly 

 striate, and of a paler colour, with occasionally a few darker 

 rays which diverge from the direction of the beaks to the 

 lower margin. 



Var. 4. cinerea. Shell larger and flatter, with fainter 

 striae. P. cinereum, Alder, Suppl. Cat. Moll. Northumb. p. 

 4 ; F. & H. ii. p. 125, pi. xxxvi. f. 2. 



HABITAT : Slow streams and standing water every- 

 where in these isles ; and it is one of our upper tertiary 

 fossils. It also ranges from Siberia to Sicily. Var. 1. 

 occurs in many of the northern, eastern, home, and 

 south-western counties of England, as well as in South 

 Wales and Cork. This is also one of our tertiary fossils, 

 and extends from Sweden to the South of France. 

 Specimens from the Swansea Canal, near some tinworks, 

 have the beaks more or less eroded, in consequence pro- 

 bably of the water containing an extremely diluted por- 

 tion of sulphuric acid, which is used in that manufac- 

 ture. The fact, which has been noticed and considered 



