EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 34/ 



over and the Goniatite stadium passes directly into the Ammonite stadium. 

 The development of the sutural line by folding of the septum advances 

 from without inwards ; on the contrary, the new lobes and the new 

 saddles are intercalated, almost always, at the lateral 

 suture of the whorl, and rarely on the external ridge. 



The second suture is distinguished from the first in 

 almost all Ammonoids by the development of an ex- 

 ternal ventral lobe, more or less deep, simple or bifid, 

 which gives rise to two external saddles caused by the 

 dichotomy of the original simple saddle. It is rare that 

 it is confined to these three elements; generally, there 

 is added besides a lateral lobe and a lateral saddle. In 

 the more simple forms the suture has by that time 

 acquired its definite shape, and all the later chambers 

 present the same design at the point of their attachment. 

 There generally occurs, however, a multiplication of the 

 lobes and of the saddles, and the external lobe takes part 

 in it by one small median saddle becoming bifid. 



Such is the characteristic development of the suture 

 in the Goniatites, the Clymenias, and a small number 

 of the Triassic Ammonites. In the Ceratites and the 

 true Ammonites there takes place exactly the same differentiation at the 

 outset as in the Goniatites; but later, when the shell has reached the size 

 of 3 mm. in diameter, begins the secondary slashing of the lobes and of 

 the saddles of the exterior and of the interior. (See O of Fig. 116.) 



At the size of 4 mm. the Ammonites are generally in possession of these 

 characteristic suture lines, which from that time on remain constant, or 

 at least suffer very slight change. In the determination of the several 

 species it is necessary to compare the suture lines of only the mature 

 forms. The external lobe does not tend to become bifid in the Goniatites 

 and Ammonites, the most ancient geologically, as in a stadium of relatively 

 tardy growth. In the relatively young Angustisellati the division into 



116 Develop- 

 ment of the suture 

 of an Ammonite 

 ( Trobites subbul- 

 latus). G = ist 

 suture, H 2d, I 

 = 3 d, L = 7 th, MN 

 sutures of sec- 

 ond whorl, O = 

 definitive suture. 

 (After Branco.) 



FIG. 117. Suture of Pinacoceras Metternichi. (After Zittel.) 



two lobes is distinctly accomplished. In a single form, or even in series 

 of forms, or in the most closely related species, the geologically younger 

 representatives generally possess the more differentiated suture lines; 

 on the contrary, however, it is not possible to deduce the geological age 

 of an Ammonite from the structure of the suture line alone. In the Trias 

 there are forms (Pinacoceras, Fig. 117) which present lobes so finely 

 slashed and so complicated that one can scarcely observe similar ones in 



