AND ORDER IN TETRABRANCHIATE CEPHALOPODS. 201 



round-backed group from four to twenty-two (Amm. Calypso D'Orb.) ; the septa of a Bacu- 

 lite, however, retain the embryonic number of four or six lobes and have precisely the same 

 form in the adult and in the young. 



This law proves that there is a direct connection between the position of a shell in the 

 completed cycle of the life of this order and its own development. Those shells occupying 

 the extremes of the cycle, the polar forms, being more embryonic than the intermediate 

 forms, although in regard to geological sequence and structural position one of the ex- 

 tremes must be of a higher zoological rank. 1 



There is, however, another and more striking correlation between the Ammonites of 

 the Jurassic era and the adult period of individual growth. All the shells of earlier 

 eras among Ammonoids, such as those of the Clymenire, Goniatites, and Ceratites, are 

 deficient in senile characteristics. The radii of their whorls do not diminish in old age, nor 

 do their septa or ornamentation show any signs of degradation ; each successive species 

 adds something to the increment of complication, and there are no visible traces of a failing 

 vitality, so far as I know, either in the individual or in the group to which it belongs. 

 But in the same manner that the individual begins to show signs of decay during the adult 

 period do the Ammonoids begin to produce individuals and species with well-marked se- 

 nile characteristics at the moment when they reach their maximum of development in the 

 Jura. Doubtless the softer parts were affected by the decline of the vital powers of the 

 individual among the earlier series, above mentioned, but the changes were not such as to 

 visibly affect the shell or the septa. 



Among the Nautiloids, also, this law appears to hold, since, as previously described, they 

 maintain the progressive complication of their septa throughout the series, and if there are 

 any signs of senile degradation they must be found among the ornamented species of the 

 Carboniferous, when the group reaches its fullest expansion. Within itself this series may, 

 as I have inferred, correspond with the changes of the different periods in an ornamented 

 individual of the Carboniferous; but with regard to the order of Tetrabranchiates it is emi- 

 nently a " progressive series." It forms the organic base of the order, as the Clymeniaa 

 form the organic base of the Ammonoids ; but while it completes the cycle of its life they 

 do not, but are cut off after existing for a limited time in the midst of their growth. The 

 Nautiloids, in fact, are dying a natural death of decay ; whereas the Cly menial, Goniatites, 

 and Ceratites were for the most part abruptly destroyed at the end of the Devonian, Car- 

 boniferous, and Triassic epochs. The concentration, also, of the whole progress of the 

 Nautiloids within the boundaries of one small group, the Clymeniae, as will be seen pres- 

 ently, confirms their progressive nature and shows them, notwithstanding their wide-spread 

 distribution, to be but little more than an equivalent of the latter. 



The first septum of Nautilus PompUius, and also of Nautilus Uneatus from the Inferior Oolite, 

 and of Nautilus bohemicus Barrande, according to D'Orbigny's figure, has an abdominal cell, 

 and the lateral portions slightly sinuous, and the siphonal aperture even at this early age 

 is central. The striations are broader externally than internally, indicating from the apex 

 the tendency towards the spiriform mode of growth. 



The presence of the siphon shows that the young does not repeat the siphouless adult 



l Thit this " higher zoological rank " also implies rela- teristies of the shell from its own young, may be seen by 

 tively a higher organization, although, as here stated, the adult referring to the last page of this article. 



may be quite embryonic and differ but little in the charac- 2 D'Orbigny, Op. cit. Terr. Jurass. Ceph., pi. 31, p. 156; 



Barrande, Op. cit. 



