No. I.] LARVAL STAGES OF SCHLOENBACHIA. 249 



shell probably included part of the spiral chamber, but for want 

 of a natural indication of the end of the stage, the phylembry- 

 onic is arbitrarily limited to the protoconch. 



Aiianepionic . — With the formation of the first septum the 

 animal is considered to cease to be an embryo and to begin its 

 larval history. This, of course, is purely arbitrary, since the 

 ammonites are all extinct and we have no way of knowing at 

 what stage they left the egg. At this period the siphon, which 

 is in the center, takes up nearly half of the height of the whorl. 

 The first septum consists of a broad, long, abdominal saddle, a 

 pair of rather narrow lateral lobes, and a pair of short, narrow 

 saddles on the umbilical shoulders. This is shown on PI. A, 

 Fig. 3, and PI. C, Fig, i. It is distinctly nautilian and corre- 

 sponds to some Silurian nautiloid genus, although it is not 

 possible to say which one, because the characters are not dis- 

 tinctive enough. The internal part of the septum is nearly 

 straight, showing no lobes nor saddles. 



Metanepio7iic. — The second larval substage begins at the 

 second septum, when the whorl is low, broad, and deeply 

 embracing. PI, C, Fig. i, shows that at the second septum the 

 broad, abdominal saddle is divided by a deep and broad ventral 

 lobe ; at this stage the shell resembles the Lower Devonian 

 Anarcestes, one of the first of the typical ammonoids. At the 

 third and fourth septa little change takes place, but these prob- 

 ably correspond to Parodoceras and Prionoceras of the Devonian, 

 At the fifth septum the ventral lobe broadens, showing a tran- 

 sition from Prionoceras to GlypJiioceras (or Gotiiatites s. str.). 

 The ananepionic and metanepionic substages take up the first 

 quarter of a coil. The siphon, during this substage, is still 

 median, and remains so up to three-quarters of a whorl, when 

 the paranepionic stage is well along, but always decreasing 

 in relative diameter as compared with the height of the 

 successive chambers. The form of the shell at the metane- 

 pionic stage is shown in the first quarter of a coil from the 

 protoconch, on PI. A, Figs. 4 and 5 ; the septa are shown 

 on PI, C, Fig, I, at the second, third, fourth, and fifth. On 

 some specimens the metanepionic substage ended with the 

 fifth septum. 



