72 CARBONIFEROUS AMMONOIDS OF AMERICA. 



shows this, and also the second chamber wall; tigs. and 10 show the ananepionic 

 suture with half a coil attached. PI. XVI, tig. 1, shows the initial suture along with 

 the later ones. While this stage can not be compared to any particular genus, it 

 corresponds to some nautilian form of the Silurian. The ananepionic siphon is aliout 

 halfway between the dorsum and the venter; in this character, too, agreeing with 

 the nautiloids. Where the siphon passes through the partition the wall is bent back- 

 ward in a cone and has a siphonal collar around the tube. The surface is still 

 smooth, no ornamentation of an\' sort ever having been seen on early stages of 

 ammonoids. 



Metanej) ionic. — W^ith the second larval substage the shell becomes a true 

 ammonoid. This begins with the second suture, which takes on the ventral lobe of 

 the goniatites. The shell is smooth, as before, and the whorl changes little in shape, 

 being still low, broad, and little embi'acing. The sutures and shape correspond 

 exactly to Anarci/ntes, the primitive goniatite and radicle of the ammonoids. Anar- 

 cestes was named but not characterized by Mojsisovics," and afterwards defined by 

 Hyatt,* as containing forms with smooth, l)road, and low whorls, with semilunular 

 cross section, deep uml)ilicus, and rather broad abdomen. Goniatiti-s t^uhiiiKitiJitms 

 Schlotheim, of the Middle Devonian, was chosen as type of the genus, liut most of 

 the species occur in the Lower Devonian, in the Hercvnian beds, which were for- 

 merlj' assigned to the Upper Silurian. 



Goniatites crenistria shows the Anarcestes stage at the second and third sutures, 

 and resembles closely ^1. hitcio'pfdtxx Beyrich of the Lower Devonian. On PI. XIV, 

 fig. 6, is seen the transition from the ana- to the meta-nepionic; figs. !» and 1(» show 

 the transition from ananepionic (first suture) to metanepionic (second and third 

 sutures); figs. 11 and 12 show the Atiarce--:tis stage at the first and second sutures 

 visible on the whorl. The metanepionic sutures, seen in projection on PI. XVI, 

 figs, lb and c, consist of a deep, rounded, ventral lobe, and a pair of broad, shallow, 

 lateral lobes. When the animal has progressed thus far in its development it is a 

 true goniatite, and the siphon has already- turned to the outside of the whorl, or 

 abdomen. 



Pamnepionic. — When the broad lateral lobes become indented with a pair of 

 lateral saddles, the sutures, the narrow umbilicus, and the broad, low whorl all 

 correspond to Parodoccras Hyatt, of the Middle and Upper Devonian. G. crenistria 

 reaches this stage at the fourth suture, at a diameter of about 0.68 mm., one-third 

 of a whorl, and continues in it for the fourth, fifth, and sixth sutures, up to a 

 diameter of 0.80 mm., and five-eighths of a whorl. PI. XIV, figs. 9 and 1<I, shows 

 the form at the Purodoceras stage, at one-half a whorl, with the following dimensions: 



Millimeter. 



Diameter 0. 74 



Height last cuil 3H 



Height last coil from the urotocunch IS 



Width of last whorl 77 



Involution 26 



Width of umbilicus 06 



" Cephalop. ^Mediterranen Triasprovinz, p. 181. 

 f-Proc. Boston Soc. Nat. Hist., Vol. XXII, p. 309. 



