438 STUDIES IN EVOLUTION 



larval shell, the outline of a section is lunate, but the pro- 

 portions of length and height are not materially changed. 

 A section of the adjacent whorl is still more arcuate, as 

 shown in figure 6, and in an adolescent specimen 11.5 mm. in 

 diameter it is seen that the diametral relations have become 

 interchanged, and that the outer whorl is elliptical in a verti- 

 cal direction, and excavated by the inner whorl to nearly half 

 its longest diameter, making the shell in all neanic and 

 ephebic stages decidedly compressed in outline (figure 13). 



The first septum (figures 1, 2, 4) is moderately concave, 

 and extends nearly to the axis. The suture is simple, being 

 nearly in a single plane, without apparent lobes or saddles. 

 Occasionally the internal mould shows a siphonal lobe due to 

 the breaking away of the extremely thin filling between the 

 siphon and ventrum, but perfect specimens determine this to 

 be an accidental condition. 



In the section represented in figure 12, the first two septa 

 are much thicker than those immediately succeeding, a fea- 

 ture also noticeable on the exterior of the internal mould. 

 Likewise the first and second air-chambers are deeper than 

 the three or four following. With these exceptions the septa 

 and air-chambers are generally uniform in their progression 

 until the adult stage. 



It has been already noted that the first septum is extremely 

 simple, without apparent lobes or saddles. In the second sep- 

 tum there is a well-developed sinus over the siphuncle, form- 

 ing a rounded ventral lobe, and a broad lateral saddle, with a 

 corresponding, though less prominent, dorsal saddle. The 

 third and following septa present more and more sharply 

 angular ventral lobes, until finally it is further extended by a 

 siphonal fissure in post-nepionic stages. The lateral saddle 

 is not so strongly curved from the fourth to the seventh 

 suture, which is quite flat, but in the eighth a slight retral 

 bend is observable. This evidently marks the inception of 

 the lateral lobe. The septum is now divided into the leading 

 members characteristic of the group ; namely, a ventral lobe 

 and saddle, a lateral lobe and saddle, a dorsal saddle and an 



