198 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



similar to Trochoceras ; the Crioceras an open coil answering to Lituites ; the Toxoceras a 

 curved tube like Cyrtoceras, and so on. Most of these are exclusively Cretaceous and, as 

 will be shown presently, are at the head of the Ammonitic series. Thus those approx- 

 imating to the normal, discoidal Nautili in form are next to them in time, and those 

 approximating to the beginning aberrant Nautiloid are the farthest removed. The Am- 

 monoids, therefore, are in every respect polar to the Nautiloids, and entirely revolutionize 

 the succession of the latter. This is equally well exemplified in the morphological sequence 

 of their structure. The Orthoceratite, on account of its simple concave septa, straight 

 cone, and priority of geological position, may be considered as the elementary form. Be- 

 tween it and the involute discoidal Nautili, we find the Cyrtoceras and Phragmoceras some- 

 what more bent, and the septa with very shallow lateral lobes; the Gyroceras with a very 

 loosely coiled shell, and the lateral lobes deeper ; Lituites closely coiled with two shallow 

 lateral lobes ; and lastly, the discoidal Nautili leading to the involute Nautilus ziczac, and its 

 angular lateral lobes and rounded cells. Among the Ammonoids, there are the ClymeniaB 

 simulating in the lowest species, such as Clymenia laevigata, the open umbilicus, almost dor- 

 sal siphon and simple lateral flexures of the Trocholites or Lituites, and in the highest, Cly- 

 menia pseudogoniatitcs, having perfect angular lateral and abdominal lobes. Next the Goni- 

 atites and Ceratites, with their more involute forms and numerous lobes and cells, and the 

 keeled Ammonites with a greater variety of forms and foliated lobes, and then the group of 

 Ammonites with round abdomens which precede the tubular aberrant genera and connect 

 them with the discoidal Ammonites. Lastly, the aberrant genera themselves, which are 

 successively more and more open from the Scaphites, as will be shown farther on, to the 

 perfectly straight Baculites. This of course is not strictly their connection, because no 

 directly traceable linear series exists, but only the general resolution of the elements of 

 form as determined by the special affinities of the septa. Morphologically, therefore, and 

 with regard to the general sequence in time and structure, the aberrant Nautiloids are to 

 the discoidal Nautili as uncoiled cones to involute cones, and the discoidal Ammonites to 

 the aberrant Ammonoids as involute cones to uncoiled cones. In other words, if we wish 

 to look at the order as a whole we may consider the straight Orthoceratite as coiling upon 

 itself and producing the discoidal shells, and these uncoiling to generate its structurally 

 antithetical but polar form, the straight Baculite. 



The senile period of the individual may be contrasted with the young, as the Ammonoids 

 were with the Nautiloids, and its polarity to the young expressed in nearly the same 

 phraseology which was used to describe the morphological inversion of the two series by 

 simply substituting the terms "old age" for "Ammonoids," "rising period" for "Nautiloids," 

 and " characteristics " for " forms." 



Thus it may be said that the old age of the individual is the opponent of the rising pe- 

 riod, expressing in every step opposition to the latter and reproducing the same character- 

 istics, but in such succession that they are removed from their prototypical characteristics 

 to the greatest possible degree both of time and organization. Thus ; the external orna- 

 ments, as previously stated, in dying out pass through a series of changes which are in- 

 versely the same as those by which they came into being, and the periods most alike are 

 the two smooth ones situated at the extremes of individual life. The polar forms are mor- 

 phologically unrolled Ammonites, and in dying out pass through a series of forms which are 

 inversely the same as those by which the Nautili came into being ; and the forms most alike 

 are the two straight cones, the Orthoceratite and the Baculite, situated at the extremes of 



