METAZOA MOLLUSCA. 241 



dence of a primitive condition, but the asymmetry of the 

 internal organs proves, as already pointed out, that these 

 forms have descended from spiral, asymmetrical Ptero- 

 pods. The shell has become adapted for swimming, 

 while the internal organs have not been so modified. 

 To strengthen these views there are species of these 

 usually straight forms which have the protoconch coiled 

 dorsally, indicating a former coiled condition. 1 This 

 genus exhibits the cone-in-cone structure plainly seen in 

 Creseis striata Rang (No. 559; PI. 560), which is large 

 and finely striated on the outside. This straight cone 

 may sometimes in the same species become horn-shaped, 

 as seen in PI. 560. 



Cuvieria colnmnella Rang is like Creseis when a larva. 

 No. 561 is a very rare specimen in which the small cone- 

 shaped young is seen preserved. When older the animal 

 builds a transverse partition across the slender tube, then 

 advances and makes a much larger tube (No. 562). The 

 young shell is usually broken off, so that only the part 

 built by the adult is seen (PI. 563). 



Cymbulia peroni Cuv. (No. 564, animal; No. 565, 

 model ; PI. 566), is a' recent form, and has an external, 

 spiral, operculated shell when a larva. This is thrown 

 off and a secondary shell forms which is also cast aside. 

 Finally in some species an internal cartilaginous shell is 

 produced which is retained throughout life. The figure 

 on the left in PI. 566 is a view of the shell of Cymbulia 

 from above with the wings expanded, and the figure on 

 the right a side view showing the slipper-like form of the 

 shell and the position of the animal within it. The 

 model (No. 565) represents the adult with its delicate 

 wings spread. Tiedemannia neapolitana (No. 567) is a 

 nearly related genus with an internal shell. 



The shell of Hyalea is essentially a hollow cone. The 



1 Chall. Rep., Zool., XXIII,. Rep. on Pteropods, part 3, 1888, 

 p. 36. 



