304 GEOLOGY OF THE ISLE OF WIGHT. 



state of molluskite. The constituent substance of 

 the shell is occasionally changed into calcareous spar, 

 and admits of a fine polish ; nodules of crystallized 

 sulphate of barytes of a pink colour, are not un- 

 common within the shells. Some examples which 

 I extracted from the bed containing the fossil trees, 

 are partially invested with lignite. One pair of 

 valves has been separated, and the hard grit with 

 which they were filled cleared away, so as to deve- 

 lop the characters of the hinge, &c. 



The shell is of an ovate form, from four to six inches long 

 three inches in altitude, and two inches in thickness or dia- 

 meter; the weight of a pair of valves cleared from extraneous 

 matter is eleven ounces. The shell is equivalved, sub-equilateral, 

 the posterior half one-fifth longer than the anterior, and com- 

 pressed along the margin of the anterior slope. It is very 

 thick and strong, and marked externally with longitudinal 

 stria'. The summit is rounded, antero-dorsal, and slightly 

 inclined forward ; the umhoncs are decorticated, as in most 

 of the shells of the Unionidaj. The ligament is external, 

 post-apicial, and marked with transverse rugae. The base or 

 circumference of the shell is entire ; the margin or internal 

 lip is thick and flat. The inner surface is smooth, with the 

 exception of a few irregular projections of nacreous deposit. The 

 hinge is dorsal, the hinge-plate very thick with a deep fosse 

 beneath the lamellar portion. The cardinal teeth of the right 

 valve are two; the anterior one is the largest, and bifurcated ; 

 the lamellar tooth-plate is broad. The anterior muscular im- 

 print is in front of the large cardinal tooth, and immediately 

 d this impression is the indentation left by the attachment 

 of the retractor muscles of the foot. The posterior muscular 

 imprint i- placed ;it the extremity of the lamellar topth-platc. 



