AND ORDER IN TETRABRANCHIATE CEPHALOPODS. 197 



the spiral, increasing the bulk of the whorls and deepening the flexures of the septa. 

 Age, indeed, seems to produce no greater impression upon these features in the individual 

 than it does in the whole group. There is every reason for supposing, also, that in its 

 other characteristics the shell agrees equally well with the decline in the whole group of 

 the ornamented species and their replacement by smooth shells. 



If the growth of the ornamented carboniferous shells agrees in any manner with their 

 representatives, the ornamented and keeled Ammonites, the adult ornaments either entirely 

 vanish or become partially obsolete in the old. We should then have in any one indi- 

 vidual three periods answering to the three periods in the general life of the series : the 

 youngest period with its bent, non-enveloping, smooth tube, and simple septa resembling 

 a Phragmoceras, or a Cyrtoceras among the aberrant genera ; the adult period with its 

 profusion of ornaments especially belonging to the epoch of its occurrence and the re- 

 turning smoothness foreshadowing the smooth shells of the last days of the entire series. 



I have not yet had an opportunity of examining any specimens of the ornamented 

 Nautili which betrayed evidences of senile degeneration ; but this is not surprising, since 

 among hundreds of Ammonites it is often impossible to pick out one specimen which pre- 

 sents a complete history of its own life. 



The Ammonoids are the opponents of the Nautiloids, expressing in every step of their 

 serial arrangement opposition to the latter, reproducing the same forms, but in such suc- 

 cession, that they are removed from their prototypes among the Nautiloids to the greatest 

 possible degree both of time and organization. For this reason, and because the word has 

 been used to designate similar phenomena by others, I shall call this general repetition 

 of form morphological polarity, whether it be in the young and old periods of the indi- 

 vidual or of the group. 



The series under consideration begins in the Devonian with the normal discoidal shells 1 

 of the Clymenia?, and mounting step by step in the complication of the septa, the multi- 

 plicity of the forms and the number of species, reaches its culmination in the Jurassic and 

 dies out with the Cretaceous. 



The number of species in the Jurassic, according to D'Orbigny, are in the aggre- 

 gate about two hundred and twenty-two, whereas those of the Cretaceous are but one 

 hundred and forty-four, making an excess in the Jura of seventy-eight species. 2 The re- 

 searches of the same indefatigable paleontologist have demonstrated that the Ammonites 

 differ from the Nautili in the suddenness with which they disappear, having, according to 

 his " Prodrome de Paleontologie," upwards of forty-two species in their last geological 

 horizon, the " Senonien." The aberrant genera, on the contrary, first announce themselves 

 with two species of straight shells of the genus Bactrites in the Devonian, and progressing 

 very slowly do not arrive at the maximum number of species until the close of their 

 career in the " Senonien." Out of eighty-six in the Cretaceous, about fifty, or more than 

 one half, are confined to this formation, and exceed the normal forms only by some eight 

 species. Many of these aberrant genera are precise repetitions of the aberrant Nautiloids, 

 which in the Silurian were, also, numerically superior to the normal Nautili. The Baculite, 

 for instance, is a straight cone like the Orthoceratite ; the Turrilite is a turbinated form 



l There can be but little doubt of the affinity of the Clymenian, has well-defined abdominal and lateral lobes, 



Clymenia; with the Ammonoids, since the discovery of the which are undeniable Ammonitic characteristics. 

 Clymenia pseudogoniatites (Guido Sandberger, Beobacht. a D'Orbigny, Prod, de Paleontologie, Vol. III., 1850. 

 iiber d. Clym. Wiesbaden, 1853) This species, an undoubted 



