CAECUM. 327 



It would appear that the shells of Vermetus and Caecum are 

 more similar than those of Siliquaria and Caecum, 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 branchiae,, it has, in the marked dorsal carinse of 

 the animal, an equivalent branchiferous canal ; the foot is also 

 more complete in Siliquaria than Vermetus, and in that re- 

 spect is nearer Caecum. Considering all these points, I think 

 Caecum has strong claims to be a component of our new 

 family of the Vermetidae, and I associate Turritella with it, 

 for the reasons given below. 



C. TRACHEA, Montagu. 

 C. trachea, Brit. Moll. iii. p. 178, pi. 69. f.4; (animal) pi. K.K. f. 1. 



I discovered the animal of Caecum 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, Caecum 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, fitting 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 in 

 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 buccal mass is distinctly visible, the corneous plates 



