206 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



from their contracted apertures, and spreading out after each contraction into a broader 

 whorl, which again contracts, and so on. By this process the spiral is reduced to a succes- 

 sion of circular segments, and the symmetry of the logarithmic curve, in which the discoi- 

 dal forms usually revolve, entirely destroyed. Each segment tends to strike off at a tan- 

 gent from the main direction of the spiral, but is turned back again toward the centre of 

 revolution at every new season of growth. Amm. bulletins caps this series, and the spiral 

 is so deformed by the successive segments, that from the side it has a spherically triangular 

 aspect. The old age features of Amm. Humphries/amis are in this way adopted even at a 

 young stage in Amm. microstoma, and eventually obliterate the regularity of the spiral in 

 Amm. bullatus, because the old age degeneration of the whorl in the first is but a permanent 

 chamber of the same general character with the successive permanent chambers of the 

 two last members of the series. 



Amm. terebratus of the Oxford Clay stands intermediate, geologically and zoologically, be- 

 tween these and the aberrant genus Scaphites of the Cretaceous. The last chamber in which 

 the animal lives bends upon itself, making an acute angle with the preceding portion of the 

 whorl ; and since these chambers are probably present at each arrest of development and 

 reabsorbed, as in Scaphites, when the season for growth again returns, the likeness to the 

 latter is perfect. The curve of the last chamber is much within the prolongation of the 

 spiral, and shows the same tendency that was perceived in the growth of Amm. microstoma 

 to depart from, and then return toward, the centre of revolution. 



The Scaphites come next, and with them the latter part of the whorl actually does strike 

 oft' at a tangent, but they nevertheless obey the same law, and are forced at last to turn upon 

 themselves toward the centre, forming a kind of shepherd's crook. The whorl is usually 

 more or less flattened, until near the mouth, where it gradually lessens in size and becomes 

 more tubular. They may have upwards of ten lobes as in the adult [Scaphites const rictus). 

 Ancyloceras has the whorl tubular throughout, but generally flattened somewhat laterally, 

 and the tangential departure of Scaphites is not only greatly exaggerated in the shepherd's 

 crook of the adidt and old, but its effect can be observed in the earliest periods of some 

 species, such as Ancyloceras Matheronianus D'Orb., their whorls being separated from the 

 very beginning. None of the species have more than six lobes. The whorl of Hamites has 

 a rounder outline, as a rule, than in Ancyloceras, and the departure is so excessive that 

 the spiral is lost in a succession of straight tubes, making a double shepherd's crook, and 

 there are never more than six lobes to the septa. 



In Ptychoceras the shell is yet farther reduced to a single straight shaft with one crook, 

 and in Baculites even this is gone, the spiral tendency of the growth not having sufficient 

 power to produce even the approximation to its former mode of revolution still evident in 

 Ptychoceras. The straight tubular cone, with its usual complement of four to six septal 

 lobes, is all that remains. 



The senile tendency of Amm. Mumphriesiatius has gradually culminated in these succes- 

 sive growths until the degeneration of the septa and shell which were primarily indicated 

 in the Inferior Oolite have terminated, as we have previously remarked, in completely un- 

 coiled and rounded whorls and in a number of lobes reduced to the embryonic formula of 

 four or six. 



Notwithstanding the accuracy with which these correlations may be followed upward in 

 time throughout the Ainmonoid series, they do not necessarily indicate any priority in 

 time for those species which primarily produce the senile characteristics, except in this gen- 

 eral series. 



