Hyatt.] 400 [Nov. 16, 



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 posi- 

 tion 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 ap- 

 proximation. Prof. W. K. Brooks' 11 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 periconcJi grow- 

 ing 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 liv- 

 ing chamber closed apically and completed by a single septum, 

 which had a eaecal prolongation reaching across the first air cham- 

 ber and resting upon the inner side of the scar. It is proposed 

 to call this stage the Csecosiphonula, since it is undoubtedly the 

 primitive stage of that organ. The csecosiphonula may indicate 

 the former existence of an ancestral form having a central axis com- 

 posed of similar closed funnels or csecal pouches. 12 



The third silphologic stage in Nautiloids was completed by a sep- 



" Proc. Bost. Soc. Nat. Hist., Aff. Moll, and Molluscoid., XVIII, 1876. 

 12 See also similar remarks by Whitfield, Bull. Amer. Mus. New York, No. 1, and 

 Embryol. Ceph. by the author, Bull. Mus. Comp. Zool., Ill, No. 5, p. 100. 



