1893.] Phylogeny of an Acquired Characteristic. 867 
wall and put in new septa behind it, together with the con- 
necting tube until it reached s’, and finally the last septum, 1. 
s. This one, l. s., was really the last one built and it formed 
the floor of a true living chamber, 1. c., formerly occupied by 
the animal at the time of its death and burial in the sediment 
of the Carboniferous period. Figs 1-2 show a similar fossil 
but with a longer, although still incomplete living chamber. 
If the external wall of shell had been preserved none of these 
structures could be seen. Figs. 5-7 show a fossil Temnochilus 
crassus, a shell of the same family with this external wall pre- 
served and all these internal structures covered up. The 
impressed zone is the re-entrant curve shown in all these fig- 
ures and especially marked in the lower outline of an outer 
whorl of another Carboniferous species, Metacoceras dubium 
Hyatt, fig. 8, im. z. ; 
Fic. 8. 
