80 BULLETIN OF THE 



evident that this, when it occurs early in life, must, as in the modern 

 Nautilus, occasion a more rapid spreading of the sides at the apex 

 than is to be found in the Striati of the Jura. This, of course, does not 

 exclude the effect of the spreading of the sides independently of involu- 

 tion, as in Nautilus excavatus Sow. of the Jura; but this is not gen- 

 erally so well marked in the young as in this species, and has no 

 bearing upon the question, which concerns only the prevalence and 

 early development in time of involution, as it may be observed in full- 

 grown specimens. 



The Loevigati of the Tertiary appear to come no nearer to the 

 existing Nautili than the Radiati, except in their smoother shells. The 

 group of Aganites or Nautili, with deep lateral lobes like those of 

 Clymenia, form a distinct genetic series, but they are no exception to 

 the rule. Nautilus aganiticus, gravesianus, and sinuatus of the Jura 

 are all less involved than either Nautilus aturi, zic-zac, lingulatus, or 

 Morrisii, and others of the Tertiary. 



Another peculiarity, the concavity of the dorsal side, is earlier de- 

 veloped in the closely coiled young of Jurassic, Tertiary, and existing 

 Nautili than in the Palaeozoic forms. Barrande, among the Nautilini 

 of the Silurian, treats this characteristic as one which is due to the in- 

 volution of the whorls. His figures show that it is very slight, and 

 arises at a comparatively late period, and only after the whorls come in 

 contact. It does not exist at all in Nautilus vetustus, which is a very 

 loosely coiled whorl. The conclusion here also seems to be inevitable 

 that a characteristic, at first fluctuating, and pertaining exclusively to 

 the older periods of growth, becomes more embryonic in the later 

 species of the same genetic series. In this instance also we have, as 

 the result of this law of acceleration, a characteristic which is at first 

 dependent upon another characteristic, the involution of the whorls, 

 becoming incorporated in the organization, and finally manifesting itself 

 independently, in the growth of each individual, before the whorls 

 touch each other. It is certainly universally present at a very early 

 period in the young cf the Jurassic and succeeding periods, though 

 absent in many of the Carboniferous forms. 



These exceptions may be said to prove the rule, for species, such as 

 N. Koninckii, in which it is absent, have therr backs convex instead of 

 concave, because the abdomens of the whorls which they involve are 

 concave instead of being convex. 



