876 Values of the Stages of Growth and Decline. 
siphon or septum. It was completed by the deposition of the 
apical plate, which sealed up the aperture of the protoconch thus 
closing the opening and cutting off communication between the two 
interiors. 
This stage can therefore be named the asiphonula or siphonless 
larva. The apex of this conch was rounded, being built out in con- 
centric circles from the contracted aperture of the protoconch, 
probably before this was plugged up by the deposition of the apical 
plate. The asiphonula was not a Cephalopod, since it had no 
central siphon, nor even a septum. It may have resembled more 
or less closely the adults of some of the ancient Pteropoda. Von 
Jhering has thought, that the characteristics of the early stages of 
Ammonoids justified a comparison between them and forms of 
Pteropoda having similar protoconchs. This was our own position 
also, but we now see, that the asiphonula was not necessarily a 
wholly pteropod-like animal. It may have retained many of the 
veliger’s characteristics, and may have more or less resembled a 
generalized type to which a Scaphopod is the nearest living approxi- 
mation. Prof. W. K. Brooks’ opinion, that the Scaphopods are 
such a generalized type and that the veliger has characters which 
can be compared with those of the adult of Dentalium ought at any 
rate to be considered here. | . 
= It is not at all improbable, that the Pteropoda may never have 
served as radicals for the Nautiloids or Ammonoids, but the latter 
may have sprung directly from the ancient Scaphopoda. ; 
The cicatrix naturally suggests comparison with the posterior 
opening in the shell of Dentalium, but if our view is the true one, 
and it represents the aperture of a protoconch, no such comparison 
can be made. The development of the conch in Dentalium is, ac 
cording to Lacaze Duthier’s researches, directly continuous with that 
of the protoconch, and the posterior opening is the result of the 
peculiar mode of growth of a primitive plate of shell which is never 
closed up. The shell, in other words, is a periconch growing around 
the body in the veliger and finally coalescing to form a tube open 
at both ends. : : 
The second larval stage in Nautiloidea was composed of a living 
chamber closed apically and completed by a single septum, which 
had a cecal prolongation reaching across the first air chamber and 
1 Proc. Bost. Soc. Nat. Hist., Aff. Moll. and Molluscoid., V. 18, 1876: 
