466 BALA MILE. 



the external surface is left: one of them which was 2^ of an inch in basal 

 diameter. Shell much depressed, spreading; colour pale purple; orifice 

 large, rhomhoidal, with a slight hollow on each side for the corners 

 of the scuta: sutures generally in part or wholly obliterated; where 

 still preserved, the interlocking toothed structure is distinct ; the 

 sheath is strongly marked by lines of growth, and is of a dark 

 brown colour. The carina and rostrum are of unusually large size 

 compared with the lateral compartments ; and this, as far as I can 

 judge, must have been the case with the younger specimens. That 

 portion of the rostro-lateral compartment which forms part of the 

 sheath, is reduced to a mere ridge. The most remarkable character is, 

 that all these old specimens (of which the smallest measured nearly 

 one inch and a half in diameter) had a flat, wide, calcareous basis, which 

 is absolutely continuous with the inner lamina of theparietes, whereas 

 in the younger specimen there was no appearance of any tendency in 

 the parietes thus to grow inflected. There can be hardly any 

 doubt that in a series of specimens some would be found with the 

 parietes first forming a flat narrow ledge round the true basal mem- 

 brane (as in the following species) ; and that in others, this ledge 

 would be wider and wider, till its edges met in the middle, and coalesced 

 into a continuous plate. 



Opercular Valves.- — I have seen these only in the old corroded 

 specimens (fig. 5 c, 5 d) : they are locked together by remarkably 

 strong articular ridges and furrows. 



The Scuta have externally two or three impressed lines or narrow 

 furrows, proceeding from the apex to the middle of the basal margin ; 

 these can be seen only in one of my specimens, owing to the degree to 

 which the valves have suffered disintegration ; but they are mentioned 

 by Conrad. The basal margin is rather short compared with the other 

 two margins. The tergal margin is remarkable from the depth of the 

 upper furrow above the articular ridge, and from the size of the promi- 

 nence (appearing like the true apex of the valve) above this upper 

 furrow. There is a hollow^ for the adductor muscle, and traces of 

 crests for both the rostral and lateral depressores. The Terga are 

 generally but little corroded, and hence the dark brown membrane with 

 which they are covered is well preserved ; the shelly matter itself is 

 also brown : there is onlv a trace of this colour in the more corroded 

 scuta : the external surface of the terga is very smooth. The spur is 

 pretty well developed, and is half as wide as the whole valve. The 

 lines of growth are upturned along the carinal margin. The articular 

 ridges and furrows are much developed. The crests for the depressor 

 muscles are extremely strong ; they depend beneath the basal margin, 

 and are remarkable (fig. 5 e) from being furnished each with fine sub- 

 crests. 



lit'n-mVs hud n, unknown. 



