26 THE SHELL. 



cut open and converted into separable valves. In the Conchifera 

 this division of the mantle is vertical, and the valves are right 

 and left. In the Brachiopoda the separation is horizontal, and 

 the valves are dorsal and ventral. The monomyarian bivalves 

 lie habitually on one side (like the Pleuronectidse among fishes) ; 

 and their shells, though really right and left, are termed " upper " 

 and " lower " valves. The univalve shell is the equivalent of 

 both valves of the bivalve. In the Pteropoda it consists of 

 dorsal and ventral plates, comparable with the valves of Tere- 

 bratula. In the Gastropoda it is equivalent to both valves of the 

 Conchifera united above. The nautilus shell corresponds to that 

 of the gastropod ; but whilst its chambers are shadowed forth in 

 many spiral shells, the siphuncle is something additional; and 

 the entire shell of the cuttle-fish and argonaut have no known 

 equivalent or parallel in the other molluscous classes. The 

 student might imagine a resemblance in the shell of the Ortho- 

 ceras to a back-bone. The phragmocone is the representative of 

 the calcareous axis (or splanchno-skeleton) of a coral, such as 

 Amplexus or Siphonophyllia. 



Spines and Sculpture. It remains to speak of the sculpture 

 and particularly of the spinous processes which adorn the shells 

 of many univalves, as well as the valves of Unio spinosus, Dione 

 liipina^Ha^ etc. It is difficult in some cases to imagine the part 

 which these play in the molluscan economy-, unless it be to 

 prevent greedy enemies from masticating them without exceeding- 

 discomfort. The formation of the spines depends upon folds of 

 the mantle-margin, and regular as is the row of the long needle- 

 like projections which ornament the lateral slope of Dione, they 

 have nevertheless been formed one at a time on either valve at 

 its margin: thus they indicate the periodicity of growth. In 

 most cases marginal characters are absorbed away by the animal 

 before commencing a new growth of its shell, bvit sometimes they 

 persist both externally and internally — thus the varices of Murex 

 are simply the thickened lips of former mouths, the recurring 

 internal projections in the whorls of Segmentina, Helix interna^ 

 etc., are the marks of former rest-periods iu their growth. 

 Another class of external markings found only upon attached 

 shells, such as Crepidula, Anomia, Myochama, etc., have no 

 connection with the animal which forms them, but result merely 

 from the plasticity of the newly formed shelly exudation, when 

 brought into contact with the surface to which they are attached. 

 Thus it is not unusual to find an Anomia covered with ribs crossing 

 its surface at right-angles, or laterally to its axis of growth in 

 consequence of having been in close contact with a Pecten. 



A numerous class of deviations from the normal spiral is found 

 in certain limpet-like mollusks as well as in Haliotis (Ixxxiii). 

 In these the spire has been reduced to a simple cone, or to a very 



