AND ORDER IN TETRABRANCHIATE CEPHALOPODS. 199 



the collective life of the order. The forms of the order and the characteristics of the iso- 

 lated shell both undergo, after their vital powers are expended, a reversion which approxi- 

 mates the aspect of the exterior of the shell and the forms of the order to their original 

 simplicity. 



Farther ; during the rising period of individual growth the septa are constantly com- 

 plicating to correspond with the equally constant increase of the radii of the whorls, but in 

 old age, as previously stated, the septa deteriorate because the whorl becomes more tubu- 

 lar. The order also during its rising period has constantly complicating septa, and in its 

 decline, when the whorls become more tubular and less involute among the aberrant gen- 

 era, these parts present marks of degradation and have fewer lobes than the involute shells, 

 not above six in all, — which is the number usually found in the young Ammonite at the 

 Ceratitic period. 



The old septa of the shell of the individual retain much of the adult complication, part- 

 ing only with those characteristics which are immediately influenced by the degradational 

 changes in the breadth of the whorl. The lobes and cells may be crowded together or 

 otherwise modified, but they still hold, even in extreme old age, to the Ammonitic folia- 

 tions, and never become smooth again as in the young, however much the whorl may be 

 rounded or otherwise brought to resemble the young. In the order, also, though the polar 

 forms so closely copy their prototypes and the septa have fewer lobes to correspond with 

 the degraded character of the whorl, yet they never lose the Ammonitic foliations, or re- 

 peat the smooth septa of the prototypical Nautiloids. 



Thus the polarities of the individual are exhibited in the same manner as those of the 

 whole order with reference both to time and mode of occurrence, agreeing even in the 

 negative features last described, in the extent to which the septa may be changed by the 

 degradation in form of the old whorl and the decadence of the entire group. These polari- 

 ties, however, are not so complete as some which may be made by contrasting the life of 

 the individual as a whole with that of the order, and thus bringing into view not only the 

 agreement of polarities but the agreement of the intermediate adult period. 



The Tetrabranchiata, as regards their progress in time, have four periods of greatest ex- 

 pansion : the first, in the Silurian ; the second, in the Carboniferous ; the third, in the Ju- 

 rassic ; and the fourth, in the Cretaceous. These are the epoch of geological history, when 

 the four separate groups had their maxima of development, as previously estimated by the 

 number of species, the multiplicity of their forms, and their profuse ornamentation. They 

 show that the groups, although parallel with each other in the general phenomena of life, 

 — namely, having a beginning in time, from which they augment in complication and in 

 numbers and then die out, — nevertheless do not perform their cycles together, but each 

 by itself, and arrive at their maxima regularly in the order of their zoological rank. The 

 aberrant Nautiloids have, according to D'Orbigny, 1 about one hundred and fourteen species 

 in the Silurian, decline to (about) sixty-four in the Carboniferous, and to (about) fourteen 

 in the succeeding strata, and are absent, as previously remarked, in the Jura. They there- 

 fore predominate over the Nautili, which amount to thirty-eight species in the Carbonifer- 

 ous, and only (about) eight species in the strata intervening between that and the Jura, 

 until they are replaced by the Nautili in the last named formation. 



The Ammonites in the Jura, as previously stated, run up to more than two hundred 



i D'Orbigny, Prod, rie Paleontologie, Vol. I., 1 850. 

 memoirs nnsT. soc. NAT. hist. Vol. I. Pt. 2. 51 



