8 



Figure. 



Umbrella Indica. PL 22. Fig. 126. Shell, showing the callous de- 

 posit of the inner surface. 



Order 4. CERVICOBRANCHIATA. 

 Branchiae ; placed within a cavity in the neck. 



The neck-gilled Gastropods present a remarkable contrast of form, but 

 are allied by obvious zoological characters. No two mollusks of a class 

 are more unlike each other in external appearance, whether as regards the 

 animal or the shell, than Siliquaria and Haliotis ; yet it will be seen that 

 the affinity between them is curious, and the transition of structure easy 

 and natural. The Slipper and Cup-and-Saucer Limpets come into this 

 order, but they are physiologically distinct from the Patella, or Limpets 

 proper, whose relationship is with the Chitons. The families into which 

 the Gasteropoda Cervicobranchiata are divided are — 



Tubispiracea. Macrostomata. Capulacea. 



Family 1. TUBISPIRACEA. 



Shell ; an irregular loosely convoluted spiral tube, with the whorls 

 detached. 



The shell of this family, though of spiral growth, resembles that of Ser- 

 pula, a genus of Annelides. It is, however, convoluted like the rest of the 

 spiral gastropods, but with this difference, the whorls are wholly detached, 

 and they are drawn out into a winding tube, like a corkscrew, without 

 forming any columellar axis. Every spiral univalve, it may be here re- 

 marked, is a modification of an enlarging tube. It is not difficult to con- 

 ceive that a plastic enlarging tube might be moulded into any kind of 

 spiral univalve, with all the appliances necessary for expansion or dilata- 

 tion and for the addition of external ornament in the form of such devices 

 as varices, tubercles, spines, scales, rings, etc. The two genera of this 

 family represent the first stage in this ideal plan of conchological architec- 

 ture. The genera, one of which is characterized by having a branchial 

 fissure throughout, are — 



Vermetus. Siliquaria. 



