EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 353 



second stage (Fig. 116, ff) shows the formation of a siphonal 

 lobe by the indenting of the primary siphonal saddle. The 

 Goniatitic modifications are seen in the sutures K, L, M, O 

 of Fig. 116, and suture O expresses the combination of the 

 Ceratitic and Medlicottian types of crenate suture ; but it is 

 the secondary lobation clearly, although in this particular 

 specimen it has not its simplest expression. 



This is the general law of ontogenetic growth as developed 

 by the authors who have specially examined these facts ; but 

 in Ammonites, as Zittel says, the Ceratitic stage is wanting or 

 passed over. This we may interpret to be due to the fact 

 that the Ceratitic type of suture alone is not expressive of a 

 stage of evolution ; but the true fact expressed by Ceratites, 

 so far as its relations to differentiation of suture line are con- 

 cerned, is its crenate or secondary lobation. This secondary 

 lobation may take place in the lobes, on the sides, or on the 

 saddles, and is a stage which, in the individual growth, is 

 quickly passed over ; the order of sequence is preserved by 

 the secondary lobation always preceding the tertiary lobation. 

 The particular part of the curved surface which first suffers 

 the secondary crimping appears to be the lobe, as is seen in 

 Trobites. 



Chronological Succession of the Characters. 2. When we 

 look at the chronological relations of this differentiation, we 

 find that the time of first appearance or initiation of the 

 several types of suture lines corresponds with the normal 

 state of differentiation of the character. That is, the Nau- 

 tilian suture line is the first to appear, in the Ordovician. 

 This continues to be the only one until the close of the 

 Silurian, when the Goniatitic suture line appears. These two 

 are the only types existing, so far as known, until we reach 

 a late Carboniferous stage the Permo-carboniferous, or Per- 

 mian when the third, the Ceratitic and Medlicottian types 

 appear, seen in the genera Sageceras, Medlicottia, and 

 Xenodiscus. But in this same geological period, in the Salt 

 range group of India, is found first appearing the form of 

 suture characteristic of the fourth or Ammonitic stage, in the 

 two genera Cyclolobus and Arcestes. Thus, before the close 

 of the Paleozoic faunas, as now defined, there is seen de- 



