GENUS TETH ACUTA. 325 



diameter, but only at its basal margin, I was at first unable to 

 comprehend, how the upper part of the shell and the orifice 

 could have acquired their proper proportional width. The 

 young shell, at its first formation, starts with an orifice so small 

 that a pin could hardly be inserted in it ; and this in many 

 individuals is never increased in diameter by the diametric 

 growth of the shell ; but in place of this 5 as the conical shell 

 is added to at its base, the whole upper part disintegrates 

 and wears away, the orifice becoming thus enlarged. We 

 thus see that the corrosion and wearing away of the upper 

 part of the shell is a necessary element in its growth. The 

 development of the radii, which in some of the species, as in 

 T. purpurascens, at first seems to be quite capricious, really 

 depends upon the fact, whether the specimens have been 

 exposed to disintegration ; for I have almost always found 

 that when the outer lamina of shell has been well preserved, 

 the radii have been developed, and the orifice has been en- 

 larged by their growth, instead of by the wearing clown of 

 the upper part of the conical shell. 



Basis. — This consists of a very thin, flat, though irregular, 

 translucent, calcareous plate, which towards the edges is 

 sometimes membranous. In T. purpurascens, the basis is 

 entirely membranous. When a portion of the calcareous 

 base is dissolved in acid, a tissue is left, composed of several 

 laminae, to which numerous bifurcating cement-ducts are 

 attached : even before dissolution, these delicate bifurcating 

 ducts can just be perceived by the aid of a simple lens. 



Mouth. — The several organs present no particular cha- 

 racters. There are generally three teeth on each side of the 

 notch in the labrum. The palpi usually have parallel sides, 

 but are club-shaped in T. purpurascens and costata. The 

 mandibles have generally four teeth, but there are five in 

 T. vitiata, and only three in T. costata. The maxillae are 

 notched. The outer maxillae are bilobed in front. 



Cirri. — The segments of the three posterior pairs usually 

 support only three pairs of main spines, but there are four 

 pairs in T. vitiata and costata : between each pair, there is 

 either a tuft of fine spines, or a single fine spine. The rami 

 of the first cirrus are unequal in length. In the third cirrus, 

 the posterior ramus is sometimes much elongated, but some- 



