AND ORDER IN TETRABRANCHIATE CEPHALOPODS. 203 



rapidly. The development varies, also, among the Ammonites, according to the adult com- 

 plication of the septa, the siphon being constantly ventral. As has been described, there 

 is a goniatitic and ceratitic stage, and these are both passed through almost invariably in 

 the first whorl, and in some highly complicated species, such as Amm. discoides Ziet., or 

 Amm. Bccchei Sow., the septa are deeply foliated in the young. 



In both the Goniatites and the Ammonites the central position of the siphon, so conspic- 

 uous among the Nautiloids and Clymenisse, is passed over, and left out in the course of 

 growth, and the development of the septa accelerated in proportion to the position of the 

 shell. Thus among the Clymenise, as a rule, it is slower than among Goniatites, and slower 

 in the latter than in the Ammonites ; although in each division there are, also, differences 

 in the rapidity with which the development proceeds, proportioned to the zoological rank 

 of the species. 



In minor series we find the same principle; thus the younger periods of Amm. hybrida, 

 covering about two thirds of the spiral, have the large, squarely set ribs of the full-grown 

 Amm. plamcosta, but the adult is remarkable for its flattened sides and Beechean ribs and 

 tubercles. Next in serial order is a species, Amm. appressus nobis, MSS., 1 which has the 

 same Planicostan markings in the young, but they do not take up more than about one 

 third of the spiral, and the rest of the shell is adorned with true Beechean ornaments. 

 This species is absolutely smaller than Amm. hybrida, and the shell more involute and 

 flatter. 



Finally, we have Amm. Henleyi, with a very faint Planicostan aspect at a much earlier 

 period, caused by the prominence of ribs crossing the abdomen and nearly the entire spiral 

 with the Beechean ornaments, and finally Amm. Beechei itself, devoid of all resemblance to 

 Plamcosta. It is smooth and round at the youngest period, and in course of growth as- 

 sumes the tubercles, the narrow bifurcating ribs and flattened sides of its species, wholly 

 ignoring the Planicostan stage of the lower members of its own series. This series is one 

 among the Ammonites, in which I have observed no marks of decadence ; all is progress, 

 and the phenomena are precisely the same as those brought out in the earlier eras of the 

 rising period of the structure of the Ammonoids. The young of higher species are thus 

 constantly accelerating their development, and reducing to a more and more embryonic 

 condition or passing entirely over the stages of growth corresponding to the adult periods 

 of preceding or lower species. 



In other words, there is an unceasing concentration of the adult characteristics of lower 

 species in the young of higher species, and a consequent displacement of other embryonic 

 features which had themselves, also, previously belonged to the adult periods of still lower 

 forms. 



This law, applied to such groups as have been mentioned, produces a steady upward 

 advance of the complication. The adult differences of the individuals or species being 

 absorbed into the young of succeeding species, these last must necessarily add to them 

 by growth greater differences, which in turn become embryonic, and so on ; but when 

 the same law acts upon some series whose individuals alter the shell in old age, precisely 

 the reverse occurs, and a general decline takes place. The old age characteristics, in due 

 course of time or structure, become embryonic, and finally affect the entire growth and 

 aspect of the higher members of the series. 



The Arietes of the Lower Lias, arranged according to their zoological affinities, have first 



1 This species will be published in the Museum Bulletin. 



MEMOIRS BOST. SOC. NAT. HIST. VOL. I. Pt. 2. 52 



