CAiCUM. 327 
It would appear that the shells of Vermetus and Cecum are 
more similar than those of Siliquaria and Cecum, whilst the 
animals of the two last have the greater correlation. Though 
Caecum has not the dorsal fissure in the shell for the water to 
arrive at the branchiz, it has, in the marked dorsal carine of 
the animal, an equivalent branchiferous canal; the foot is also 
more complete in Stliquaria than Vermetus, and in that re- 
spect is nearer Cecum. Considering all these pomts, I think 
Cecum has strong claims to be a component of our new 
family of the Vermetide, and I associate Turritella with it, 
for the reasons given below. 
C. rracuEea, Montagu. 
C. trachea, Brit. Moll. iii. p. 178, pl. 69. f.4; (animal) pl. K.K. f. 1. 
I discovered the animal of Cecum trachea, in the year 1834, 
in the coralline zones off Exmouth: notes were then made on 
it, but only communicated to a few friends, and I am not 
aware that any author has mentioned the animal since that 
time, except in doubt as to its character and position. 
Having recently, at the same place, examined many specimens 
of this curious and minute species, I am enabled to give a 
particular description thereof, as well as some account of the 
still more minute congeneric species, Cecum glabrum. 
The shells of these animals have, from their forms, long 
been located with the Dentalia, but it will appear that in 
respect of the animal they have little connection with them ; 
they have also had other places assigned to them, and mala- 
cologists are still in doubt with regard to their natural posi- 
tion. ‘I therefore think this account may assist to determine 
the proper “ locus standi”’ of these mollusca. 
Animal cylindrical, arcuated, externally pure white. The 
mantle is very thick and fleshy, fittmg the shell closely, and 
not extending beyond its anterior margin. The body is elon- 
gated and slender, with a long, flat, cloven muzzle with fine, 
close, contractile, annular ridges, which on all occasions is m 
advance of the foot, and appears to assist in locomotion. The 
fissure of the mouth is vertical, and from the tenuity of the skin 
the pale red buceal mass is distinctly visible, the corneous plates 
