202 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



condition of some of the lower genera, such as Endoceras. There is, however, a faint refer- 

 ence, perhaps, to such species as Tretoceras bisiphonatum, caused by a minute dorsal lobe in 

 the young of N. Pompilius, which from its position and general appearance might be homol- 

 ogized with the dorsal chambers of that species. 1 The plainly convex septa, equable stria- 

 tions, and straight mode of growth of the Orthoceras is also omitted, the young shell being 

 wholly devoid of resemblance to these simplest forms. It is presumable that in their ear- 

 lier periods the ova have characteristics in common ; but certainly after these egg phases 

 are past, if there are resemblances between the embryonic Nautilus and the adult Endoce- 

 ras or Orthoceras, they are too transient to make any impression either upon the mode of 

 growth, the septa, or the curvature of the shell. 



Entering upon the Ammonoid series, the first member of it, the Clymenias, confined to a 

 single geological epoch, the Devonian, centres within the boundaries of one small series of 

 shells like Cbjmenia laevigata, that are separable from Trocholites only through their external 

 outline, which is flatter or more definitely discoidal than the spiral of the latter, and the 

 Cbjmenia pseudogonicdites, which we ascertained to be a true Ammonoid. 2 The range of the 

 complication is greater, so far as the septa are concerned, than that traversed by the Nau- 

 tiloids in almost the whole of geological time from the Silurian to the Tertiary. A still 

 more decided concentration is exhibited in the last named species, every individual of 

 which, according to Guido Sandberger's figures, begins life with the simple septa of a Cly- 

 menia kevigata and crowds all the essential steps of the entire advance into its younger pe- 

 riods. Whether the Clymenia? are true Ammonoids or not is of no material consequence ; 

 certainly, in one division, and, finally, in the young of one individual of that division, and 

 in a proportionately shorter time, they bring together by successive concentrations the re- 

 sults of the progress of the Nautilian septa, and proceed in the adult to make a yet greater 

 advance in another direction towards the more highly constituted Ammonoids. 



The Goniatites do not repeat in their young the position of the siphon in the Clymenife, 

 although the septa may be quite as simple as they are in Clymenia laevigata. It is possible 

 that the siphon may alter its place in the young of such simply septate species as Goniatites 

 marccllensis Van., but this is exceedingly doubtful, since in species as simple as Goniatites 

 discoideus Hall, the siphon is ventral at an early period of growth. The range of complica- 

 tion is greater than among the Clymenioe, being from Goniatites marcellensis with two shallow 

 lateral lobes to Goniatites ceratitoides, which closely approaches the Ceratites in the shape 

 and number of its club-shaped lobes. The development of the septa accords with the 

 growth of the same parts in the Clymenioe, since among the lower species it takes a longer 

 period to bring the lobes and cells to their full development than in the higher. Thus in 

 Goniatites marcellensis the septa have about the same embryological curves throughout ; in 

 G. lamed Sand., a more complicated species with two angular lateral and one large divided 

 ventral lobe, they take on the adult aspect during the second or third whorl, having passed 

 through a stage equivalent to the adult period of G. marccllensis during the first whorl ; and 

 in G. crenistria Phill., a still more advanced species, the changes are introduced even more 



1 This homology seems to confirm Mr. Sailer's opinion, that Nautiloids, with C. pseudor/oniatites, through such species as C. 

 the dorsal (his ventral) chamber is wholly independent of the binodosa, subl&vis, or costcllata, all of Munster, which have 

 siphon. (Salter on Tretoceras. Journ. Geol. Society, Lon- the angulated Clymenian septa in the adult, but are like the 

 don, 1858.) adult C. Icecigata in the young, both in the external striations, 



2 I connect Clymenia lecvigata, which, together with its al- form of the whorl, and curvature of the septa, 

 lied species, has been placed by several authors among the 



