194 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



other by the external characteristics of the shell : 1 namely, the first period, or " periode em- 

 bryonnaire," during which it is smooth and the abdomen 2 round ; the second period, or 

 " premiere periode d'accroissement," which is marked by the advent of the tubercles, or ribs 

 and keel, if there are to be any upon the adult shell ; the third period, or " derniere periode 

 d'accroissement," during which the tubercles, or ribs and the keel, are fully developed, and 

 the whorl takes on its adult configuration ; the fourth period, or " premiere periode de 

 degenerescence," during which the ribs or tubercles begin to separate more widely and 

 become depressed ; and the fifth period, or " deuxieme periode de degenerescence," when 

 all these ornaments are obsolete, and the exterior is smooth again as in the young. 



The r-ecapitulation in which he sums up the results of this remarkably original and 

 unique series of observations is equally truthful and instructive. The following paragraph 

 conveys the sense of the original, though the piquancy and force of the French is lost in the 

 translation. 



" These modifications are not due to chance, but to decided and regularly occurring 

 periodical metamorphoses, which affect the larger number of the Ammonites, and which 

 invariably operate in a regular order of succession. In fact, each one, although smooth in 

 the youngest period, covers itself at a later time in the course of growth with tubercles 

 around the umbilicus, afterwards with ribs, striations, or tubercles, upon the back (abdo- 

 men). It is then in the adult stage. Having arrived at the maximum of external com- 

 plication, all these ornaments begin to show signs of alteration ; it degenerates ; its stria- 

 tions and dorsal (abdominal) ribs first disappear ; then follow its lateral ribs or tubercles, 

 and in old age it becomes fully as simple externally as it was during the embryonic 

 period." 



The accomplished author of the " Paleontologie Franchise," however, did not extend his 

 researches to the internal organization beyond a merely casual notice of the simplicity of 

 the lobes of the septa in the young ; and, also, positively denied that these parts were 

 affected by old age : " ne montrent qu' une complication toujours croissante et jamais de 

 degenerescence." 3 



Robert Owen, the great comparative anatomist, states that the young Ammonite has an 

 embryonic stage of development, when its septa resemble a Ceratite, 4 but besides this, 

 three more periods can be traced, — one in the young, one in the adult of all shells, and 

 the other in the old age of a limited number of forms, making altogether four periods of 

 septal growth. 



The first period, corresponding with D'Orbigny's " periode embiyonnaire " of the ex- 

 ternal characteristics, has simple septa curved like those of Gonicdites MarceUensts, or any 

 of the Nautilini, and the siphon occupies an abdominal position, giving rise to an exceed- 

 ingly shallow abdominal lobe. The second, the equivalent, with reference to the more 

 complicated adult septa, of the second period of D'Orbigny in the development of the ex- 



1 Alcide D'Orbigny, Pal. Francaise, Terr. Cre't. Ceph., p. dans les dernieres eloisons qu' une Ammonite a forme'es avant 

 377, sa mort, et parait etre la suite d'une maladie ou de la vieillesse." 



2 D'Orbigny calls the outer part of the whorl the back, but 4 Robert Owen. Paleontology, p. 103, 2d ed., 1861. The 

 in common with Pictet and Agassiz I would regard it as the same authority (p. 99) also states, that " the sutures of an 

 abdomen. Ammonite are at first very slightly lobed, and become pro- 



3 D'Orbigny, Op. cit. p. 396, 2°. On the preceding page, gressively more complex; so that specimens of the same 

 however, he partially contradicts this assertion by doubtfully species have been referred to three genera — Goniatites, Cer- 

 referring certain changes of the septa either to sickness at the atites, and Ammonites, according to their age." 



approach of death, or to old age, " Cc cas a lieu ge'neraleinent 



