212 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



The ananepionic septum (PI. XXVII, fig. i) consists of a 

 narrow, undivided, abdominal saddle, and a short lateral 

 saddle; the internal portion of the septum is gently curved, 

 and gives little indication of the lobes and saddles that 

 appear on the second chamber-wall. The only part of the 

 shell that can with certainty be assigned to the ananepionic 

 period of growth is that lying between the first and second 

 septa, but that is probably not all of this second living 

 chamber. The outer nacreous shell is smooth and devoid 

 of all ornament until the end of the first coil, hence it seems 

 likely that the ananepionic body-chamber extended through- 

 out this coil, the end of which is marked by a distinct con- 

 striction, and beginning of sculpture, as seen on PL XXIV, 

 figs. 3-9. While this portion of the shell became chambered 

 in later larval stages, it was a spiral unchambered coil 

 during the first free stage of the animal. This is true not 

 only of this species of Placenticeras, but also of the early 

 larval stages of every ammonite yet seen by the writer, 

 embracing typical genera from the Carboniferous, Trias, 

 Jura, and Cretaceous, retrogressive and progressive forms 

 alike. 



Metaiicf ionic . — Following the usage of Hyatt, the middle 

 larval stage is considered to have begun when the shell has 

 assumed ammonoid characters ; this happens with the 

 formation of the second septum, and continues as long as 

 only simple goniatite characters are seen. In the older 

 ammonoids the second septum always has an undivided 

 ventral lobe distinctive of the Nautilinidas and their imme- 

 diate descendants; but in the later and more specialized 

 ammonites the second septum already has the ventral 

 lobe divided by a siphonal saddle, so that the record 

 of the Nautilinid^ is lost from the shell. The metane- 

 pionic stage is shown on PI. XXIV, figs. 3-7, and its 

 septa on PI. XXVII, fig. 2. The divided ventral lobe, the 

 lateral lobe and that on the umbilical shoulder, along 

 with the broad low whorl and smooth shell, all remind 

 one strongly of Glyphioceras, a genus diagnostic of the 

 Carboniferous; because of this resemblance, and because 

 some member of the Glyphioceratida? may probably have 



