AND ORDEE IN TETRABRANCHIATE CEPHALOPODS. 195 



ternal ornaments, and of the Ceratitic period of Owen, has a deeper abdominal lobe, and the 

 lateral portions of the septa bent into a number of pectinated, club-shaped lobes like those 

 of Ccratites nodosus. This period is not invariably coincident with the first appearance of the 

 tubercles ; on the contrary, it often precedes them, occurring while the shell is still smooth, 

 as in Amm. Kridkm D'Orb., or Amm. Beechei Sow. It is rare, however, to find a shell in which 

 the septa have attained to any considerable degree of complication before the beginning of 

 the tubercular period. The third period is always partly coincident with the third period 

 of the external ornaments. It includes the septa from the time they accpuire their Am- 

 monitic lobes until they begin to degenerate in old age, and usually accompanies the orna- 

 ments through the entire course of their adult complication. The fourth period is not 

 always distinguishable ; thus Amm. heterophyllus, Amm. primordialis, and the like, which ha- 

 bitually increase the radii of the spiral throughout life, present no very decided marks of 

 septal degradation in the oldest parts of their shells. In those species, however, such as the 

 Planulati and Macrocephali, in which the radii of the spiral in the old decrease, the size of 

 the whorl also diminishes, consequently crowding the lobes together, narrowing the breadth 

 of the cells and preventing the formation of new auxiliary lobes. In fact, the complexity 

 and number of the lobes seem to depend wholly upon the unceasing expansion of the whorls. 

 There is then more room for them, and they either broaden, or an additional number of 

 auxiliary lobes are added, as in the old of any of the more involute species of the Arietes, 

 Falciferi, Dentati, Fimbriati, or Heterophylli. When the recession of the spiral takes place 

 among the keeled Ammonites it may occasion the atrophy of the keel and an approximation 

 to the aspect of species having rounded abdomens, as in the example given by D'Orbigny 

 and Pictet of Amm. varicosus Sow., which loses its keel in the old, and resembles some of the 

 Capricorni (Amm. planicosta)} There is, therefore, a general agreement between the muta- 

 tions of the septa and form of the whorl, with those of the superficial characteristics of the 

 shell ; sufficient, indeed, to show that all the parts are more or less interdependent, and 

 have at least four distinct periods during which they are mutually modified, viz : two peri- 

 ods of development, one adult period, and one old age period. The fourth period of the 

 septa, when it does occur, is necessarily coincident with the fourth and fifth periods of the 

 external ornaments, since the last two invariably accompany the diminution in bulk of the 

 old whorl, and the degradation of the septa during the fourth period also depends upon the 

 contraction of the interior caused by this diminution. 



The correlations of these periodical revolutions in the life of the individual with those 

 displayed on a greater scale in the life of the entire order of Tetrabranchiates in time, are 

 wonderfully harmonious and precise. They open a vista through which the individual 

 may be viewed in a new and unexpected light ; standing side by side with its own series 

 of forms, it seems to embody the same biological law ; not only rising and declining within 

 the narrow limits of its separate existence as they do in their totality, but varying the 

 characteristics of its different periods reciprocally with the more extensive changes of the 

 entire series. 



The general agreement of the life of the individual and the life of the group are 

 evident and need no extended explanations ; but for the purpose of entering more minutely 

 into these comparisons, it will be necessary to give a summary of the principal points in 

 the history of the order. The Tetrabranchiates are specially suitable for an inquiry like 



l D'Orbigny, Op. cit. p. 295, and Pictet, Traite de Pale'ont., II. p. 6 76. 



MEMOIRS BOST. SOC. HAT. HIST. Vol. I. Pt. 2. 50 



