METAZOA MOLLUSCA. 239 



conch by the law of acceleration in development, so that 

 Tentaculites comes naturally after Orygoceras in our 

 classification. 



Hyolithes (No. 548, H. primordialis Hall), is usually 

 straight, but sometimes curved. Its aperture is triangular 

 and is provided with an operculum. The shell is divided 

 off in the interior by horizontal partitions. 



Conularia (No. 549, C. congregata Hall), has a very 

 thin straight shell with a four-sided aperture. According 

 to Ruedemann 1 Conularia is at times a sessile form and 

 is found attached to older individuals (PI. 550, fig. i, C. 

 gracilis Hall, x 2) or sometimes to foreign objects. PI. 

 550, fig. 2, exhibits the young of different ages attached. 



The full and beautiful researches of Fol 2 upon Ptero- 

 pods have given us much knowledge concerning the life 

 histories of these interesting animals as well as their 

 habits and structure. 



The eggs of most species are laid at the going down 

 of the sun, and the number produced by one individual is 

 enormous. 



When the Pteropod embryo of to-day leaves the egg it 

 possesses a shell and a velum. In some genera the shell 

 is present throughout life, while in others it is possessed 

 by the larva only. The velum disappears, and the foot 

 changes from a creeping to a swimming organ by devel- 

 oping two broad lateral expansions usually called wings, 

 but which are really oar-like fins. According to Pelsen- 

 eer these little creatures swim in a nearly vertical posi- 

 tion with the head uppermost or slightly sloping, so that 

 the foot is turned upward, while the fins move backward 

 and forward. 



The division of the group into Thecosomata and Gym- 

 nosomata is in accordance with the principle of a natural 

 classification. 



* 



1 Amer. Geol., XVII, XVIII, 1896. 



2 Arch. d. Zooi. Exper. et Gen., IV, 1875. 



