334 GEOLOGICAL BIOLOGY. 



(Fig. 114), the lobes being denticulated by secondary notches. In the Am- 

 monites (Fig. 115) the saddes also, as well as the lobes, are denticulated 

 in the most varied manner, notched, cut, or ramified, in form of branches, 

 or foliated. The curvature of the suture line, as well as the formation of 

 the saddles and lobes, takes place symmetrically in such a manner that 

 a median line in the direction of the height divides the turns into two 

 equal parts. The exterior lobe is called the external or siphonal lobe, 

 when the siphon is on the exterior side. For Leop. von Buch it is the 

 dorsal lobe, because he called this the back of the shell, but for recent 

 authors, who consider the external side to be the ventral part, it is the 

 ventral lobe. The opposite unpaired lobe is the internal lobe (or, accord- 

 ing to opinions, antisiphonal lobe, or dorsal, formerly ventral lobe). Be- 

 tween the two are found the lateral lobes and the lateral saddles, situated 

 on the body of the whorls, and the lobes and saddles concealed between 

 the line of contact of the contiguous whorls and the internal lobe: among 

 the former, the saddle which is found on the side of the external lobe is 

 the external saddle, the two following are the first and second lateral sad- 

 dles; all the others, up to the line of junction of the two whorls, are the 

 auxiliary saddles; near the internal lobe is found, generally, an internal 

 saddle, which is distinguished by its size from the other concealed internal 

 auxiliary saddles. For the lobes, the first lateral lobe is that which is 

 between the external saddle and the first lateral saddle; the following 

 one is the second lateral lobe; all the others are called auxiliary 

 lobes. 



The beautiful researches of Hyatt and Branco have shown that the 

 complicated lines of the suture of the Ammonites do not attain their normal 

 form until the animal has developed a greater or less number of the cham- 

 bers. The first sutures of all the Ammonites are always as simple as those 

 of the Nautilidae, Clymenias, or Goniatites (Figs. 112, 116); it is only little 

 by little that the undulating lines become marked by secondary notches, 

 and the complication of the line of the suture proceeds always from the 

 exterior to the interior side. The complication of the suture line which 

 augments with age, so that the young sutures, more simple in Ammonites, 

 resemble those of the geologically more ancient Goniatites and Nautilidae, 

 shows, probably, that this differentiation indicates at the same time a per- 

 fection of the organism. It is truly difficult to discover wherein this con- 

 sists. It is possible that the strongly ramified borders of the septa serve 

 to increase the solidity (firmness) of the shells; for, in general, the shells 

 of Nautilidae, provided with simple suture lines, are considerably thicker 

 than the shells of Ammonites ordinarily as thin as paper. If one breaks 

 cautiously, little by little, the enrolled shell of a Tetrabranchiate, there 

 are distinguished the first whorls, and finally also the initial chamber of 

 the whole coil. In the fossil evolute, or baculiform, shells this first cham- 

 ber is, ordinarily, abbreviated or broken, and it is extremely rare that it 

 is preserved. 



According to Barrande, Hyatt, and Branco, there are two kinds of initial 

 chambers in the Tetrabranchiates which can be distinguished by funda- 

 mental characters. In the Nautilus, and many of the paleozoic genera, 

 the initial chamber is in the form of a truncated cone, slightly arched or 

 straight, enlarged in front; upon the posterior convex wall, which termi- 

 nates the truncated cone, is observed a depressed cicatrix, linear (Nau- 



