452 BALANI1LE. 



added to above the level of the opercular membrane, and 

 hence their summits are oblique : in C. intertewtus, however, 

 the alae are laterally added to above the opercular mem- 

 brane, and their recipient furrows are likewise added to, of 

 which fact I have seen no other distinct instance in any 

 genus ; hence on both sides of the sutures, in the sheath of 

 this species, the lines of growth are upturned. In some 

 much disintegrated specimens, both of C. stellalus (var. 

 depressus) and of C. antemiatus, the radii have been corroded 

 away, and the diametric growth is effected exclusively by 

 the growth of the alae, which are moreover much exposed, 

 and rendered conspicuous. The sheath descends a mode- 

 rate distance down the shell. When a shell is boiled in 

 potash, the sutures (excepting when abnormally calcified 

 together, as very often happens with some species) always 

 fall apart, showingthat theunion is simply by animalised matter. 



Basis. — The basis is always membranous ; but we have 

 seen, in C. intertextus, that the walls form a flat ledge all 

 round the base, and that in old specimens of C. Hembeli, they 

 grow so far inwards and become so completely confluent, that 

 they might most easily be mistaken for a true calcareous basis. 

 I may add, that in one elongated specimen of C. stellaius 

 from La Plata, the walls had likewise grown rectangularly in- 

 wards, forming a flat base, and had then turned upwards in 

 the middle, forming a medial crest, with the edges not 

 quite calcified together. The true basal membrane is 

 very obscurely divided into concentric slips. I observed 

 in several species, attached to the lower surface, an ex- 

 cessively fine network, quadrangular or hexagonal, of 

 yellow vessels, which seemed insensibly to pass into the 

 sheets, discs, and globules of cement, by which the mem- 

 branous basis adheres to the supporting surface. I saw, in 

 C. antennatus, numerous irregular, bifurcating, and inoscu- 

 lating cement-ducts, of unequal diameter, often crossing 

 each other, and sending off branches ending in points: the 

 older ducts, instead of being solidly filled up with cement, 

 were only divided by septa. I did not succeed, in any 

 species, in discovering the cement-glands. 



Mouth. — The lahrum is slightly bullate, with the middle 

 portion depressed, but not forming a notch ; in some species 



