1887.] 401 [Hyatt. 



turn (the second in the apical part of the shell) having an open 

 funnel extending apically and joined to a loose textured siphonal 

 wall. which reached down into and lined the caecum, thus forming 

 a secondary closed tube. In accordance with the structure this has 

 been named the Macrosiphonula. 



The protoconch was present in Ammonoids and also in Belem- 

 noids, but in both of these orders it was calcareous. The tendency 

 to form a calcareous shell, which first appeared in the apex of the 

 conch of the asiphonula in Nautiloids, became by concentration of 

 development inherited earlier in the Ammonoids and Belemnoids 

 in the veliger stage, thus transforming what would otherwise have 

 been a horny shell into a calcareous one. The protoconch was, 

 however, not otherwise changed in external aspect and retained the 

 usual egg-like shape of the univalve veligers of the Cephalophora. 

 As in the protoconchs of other similar veligers of Gasteropoda, 

 etc., and as a result of calcification, the protoconch became fused 

 with the apex of the conch more intimately than in Nautiloids. 

 In other words the asiphonula, after transmitting a portion of its 

 characteristics to the typembryos of the Ammonoids and Belem- 

 noids, disappeared, having been replaced by the caecosiphonula. 

 The septum of the csecosiphonula was consequently also inherited 

 earlier, and became a functional substitute of the apical plate serv- 

 ing to close the aperture of the protoconch, and its caecum ex- 

 tended into the upper part of the otherwise empty protoconch, in 

 place of occupying the first air chamber as in Nautiloids. This is 

 a remarkable example of the law of concentration, but by no means 

 exceptional. The fourth larval stage of the Nautiloids was com- 

 pleted by the building of the third septum. This septum had a 

 long funnel and attached porous wall, but the wall formed a true 

 siphonal tube opening apically into the next section, the macrosi- 

 phon. This was the beginning of the small siphon and can be ap- 

 propriately termed the Microsiphonula. The microsiphonula was 

 the typical stage of nearly all the known genera of Nautiloids, 

 beginning with the Orthoceratites of the Cambrian and found at 

 the present time in Nautilus, and also in all Ammonoids and Bel- 

 emnoids without exception. 



Fortunately the genesis of both macrosiphonula and microsi- 

 phonula can be traced in the adult forms and silphologic stages of 

 some well-known fossils. Crytocerina had a siphon which was 



PROCEEDINGS B. S. N. H. VOL. XXIII. 26 MARCH, 1887. 



