OF MOLLUSCA. 169 



cated, and there is often a small, central, prominent tube, like an 

 internal shell formed within the old one. This state has been called 

 a species, Bentalium labiatum, Brown. 



Suborder V. Cervicobranchia. 



Gill single on the left side of the gill-cavity on the back of the 

 neck. Shell conical, symmetrical. Teeth in a few longitudinal 

 series, forming arched cross series of two or three pairs of teeth 

 (p. 167). 



Fam. XI. TECTUBIDjE. 



Teeth in six longitudinal series, the two inner close, uniform, the 

 two lateral on each side in an oblique cross line, inner one often 

 larger. Gill single, on the side of the back of the neck. 



* Mantle edge bearded or simple, without any lamina on the inner 

 surface near the margin. 



1. Nacelia. 



Shell ovate, conical, thin ; apex subanterior, near the front mar- 

 gin ; mouth simple. Teeth in six longitudinal series and in arched 

 cross series, the two inner close, the two others lower down and 

 parallel to each other. 



I. N. mytiloides. 



2. Tecttjra. 



Shell conical, convex ; apex subanterior ; margin even, simple. 

 Gill one, placed on the left side of the head, prominent externally. 



1. T. parva, t. 70. f. 7, t. 110*. f. 7. 



1 2. T. tessellata, t. 70. f. 6. 



3. T. testudinalis, t. 268. f. 2 (t. a. a. f. 2). 



4. T. cassis, t. 114. f. 8. 



5. T. flammea, t. 114. f. 5. 



6. T. fragilis, t. 114*. f. 7. 



7. T. striata, t. 114*. f. 6. 



8. T. stellaris, t. 114*. f. 4. 



9. T. conica, 1. 114*. f. 2. 

 10. T. punctata, t. 114*. f. 3. 



II. T. ?, t. 110*. f. 9. 



12. T. zebrina, t. 70*. f. 7. 



The arcuated branchial plume of Tectum parva issues from the 

 back of the neck ; in a state of quiescence it does not appear long, 

 but can be greatly extended and contracted, like a pair of ladies' 

 cross-barred forceps ; it is composed of a pectinated leaf of almost 



