26o 



ABSORPTION OF INTERNAL PARTS— DECOLLATION chap. 



the spire lose their spiral form, and take the shape of the cavity in 

 which they lie. Amongst the genera in which this singular proc- 

 ess takes place are Nerita} Olivella^ and Cypraea 

 amongst marine forms, and nearly the whole of 

 the Auriculidae^ (Fig. 168). Conns reduces the 

 internal subdivisions of the spire to extreme thin- 

 ness. It is noticeable that these genera are all 

 of considerable thickness of shell, and it is per- 

 haps the result of the whole energy of the animal 

 being directed to the formation of its external 

 protection that the internal walls of the spire 

 become atrophied and eventually disappear. 



Decollation. — In certain genera, when the 

 shell becomes adult, the animal ceases to occupy 

 the upper whorls, which accordingly die and drop 

 off, the orifice at the top having meanwhile been 



Fig. 1G8. — Auri- 

 cula Judue 

 Lam., showing 

 the disappear- 



ance of the par- (^]^Qsg(j "by ^ shelly deposit. Such shells are termed 



titions of the j j l- 



whorls, which decollated. In some land genera decollation is 



hrdXd'nnes^ the rule, e.g. in Cylindrella (Fig. 169), Eucalo- 



(After Fischer.) dium, and Biunina, as well as in many species of 



Fig. 169. — a, Decollated (adult) 

 form, and B, perfect (young) 

 form of Cylinclrellci nohilior 

 Ad., Jamaica; the dotted line 

 shows where decollation takes 

 place. 



Fig. 170. — Development of Coecum : A, 

 showing the gradual formation of 

 septa; a, apex; ap, aperture; ss, 

 first septum; s's', second septum. 

 (After de Folin.) B, Adult form of 

 C. eburneum Ad., Panama, x 8. 



the brackish water genera, Truncatella^ Cerithidea, and Quoyia. 

 Stenogyra (^Rumina) decollata, a common shell in the south of 



1 Mr. B. B. Woodward has recently pointed out (P. Z. S. 1892, p. 528) a very 

 remarkable method of shell absorption and growth in Velates and certain other 

 Neritidae. 



2 The only exception appears to be Pedipes, while in CassiduJa and Scarabus 

 the absorption is partial (Crosse and Fischer, Journ. de Conch, xxx. p. 177 f.). 



