EXTRINSIC CHARACTERS PROGRESSIVELY MODIFIED. 



which it should appear according to the law of genetic evolu- 

 tion of the characters of the race. 



Description of the Characters of the Ammonoids. In order to 

 place before the reader a concise description of the characters 

 of Ammonoids, the definitions of Zittel may again be followed, 

 furnishing as they do the precise characters needed for an 

 understanding of the problem under discussion. 



Zittel's definition of the characters of the Ammonoidea is 

 as follows : 



SECOND SUBORDER: AMMONOIDEA. Shell generally enrolled or spiral, 

 discoidal, more rarely spirally coiled, evolute, arched or straight; open- 

 ing simple or furnished with lateral and ventral prolongations. Suture- 

 line undulating, notched or with slashed or dentate lobes and saddles; 

 siphuncle cylindrical, always marginal, without internal deposit; initial 

 chamber spherical or ovoid, frequently an aptychus or anaptychus. 



In the description of the fundamental characteristics of 

 the sutures and their development we follow Zittel's synopsis. 



The embryonal chamber (nucleus, ovisac) of the Ammonoids has a- 

 spherical or transversely ovoid shape (Fig. 112, a); it is smooth, separated 

 by a contraction from the rest of the shell, and always enrolled spirally 

 about an imaginary axis. Its anterior aspect is, in consequence, essen- 

 tially different from its lateral profile, its sides 

 having a projection in form of an umbilicus. The 

 embryonal chamber, of which the height varies 

 from 0.3 to 0.7 mm., is limited in front by the 

 primary septum. The constitution of the first 

 suture gives, according to the beautiful researches 

 of Branco, excellent basis for classification. In 

 the most ancient Ammonoids it forms a straight 

 line, more or less simple, and then resembles the 

 first suture of Nautiloids; Branco calls these 

 forms the Asellati (Fig. in, A) 



In a second group the first sutural line pro- 

 ceeds forward to form an arch towards the ex- 

 terior, and forms a large simple ventral saddle, FIG. in. Ventral views of the 

 Latisellati (Fig. in, B\ edges of the embryonal 



chamber, representing^ the 



The third group is distinguished by the rela- primary septum_of an asel- 



tively narrow ventral saddle, on each side of 



which is developed a lateral lobe, and generally and C of an angustisellate 

 also a small lateral saddle, Angustisellati (Fig. gute? Bianco.) (Phyloceras) - 

 in, C). 



While the first suture of all Ammonoids is comparatively simple, more 

 or less considerable complication is produced by the later development of 

 the shell. Only a few of the more ancient types possess a sutural line 

 altogether simple, like that of the Nautilids. Almost always, even in 

 Paleozoic forms, the suture attained at least the Goniatite stadium, that 



