254 FOOT-NOTES TO EVOLUTION. 
Tornoceras retrorsum Buch is figured on Plate IV, Fig. 8. 
At the seventh chamber, three fourths of a whorl, and 
diameter of about 0.85 millimetre, the shell changes its 
form rather suddenly, the umbilicus widens, the body 
chamber narrows, and the number of lobes and saddles 
increases; this stage corresponds to the Upper Devonian 
genus Prionoceras, as shown on Plate I, Figs. 11 and 12; 
and more advanced on Plate II, Figs. 2-6, the resem- 
blance to that genus being perfect in everything except 
size, and any naturalist would have described these 
stages as Prionoceras if they had not been taken out of 
the inner coils of Giyphioceras. The septa of this stage 
are shown on Plate IV, Fig. 1, eighth septum, and Figs. 2, 
3, and 4; Fig. 5 shows the transition to Glyphioceras, 
and the corresponding shell is shown on Plate II, Figs. 
7 and 8. The adult is shown on Plate II, Fig. 9, and 
Plate IV, Fig. 6. 
DEVELOPMENT OF SCHLOENBACHIA. 
Schloenbachia begins its development almost where 
Glyphioceras leaves off, or rather it hastens through the 
stages before the Glyphioceras stage so rapidly that they 
are almost unrecognisable. On Plate III, Fig. 3 shows 
the embryonic protoconch, and the first six septa drawn 
as if unrolled, in which short space it hastens through 
the stages corresponding to Axarcestes, Tornoceras, 
Prionoceras, and becomes a Glyphioceras. Fig. 4 shows 
the shell in that stage, and Fig. 6 shows the correspond- 
ing septa. Fig. 7 shows the transition to the Gastrioceras 
stage, the septa of which are seen in Fig. 8. The next 
stage corresponds to the Carboniferous genus Paralego- 
ceras, Figs. 9 and ro, and with this the goniatitic larval 
period ends. The first adolescent (ammonitic) character 
that appears is a keel, at the diameter of 2.7 millimetres, 
and shortly after this the first lateral saddle becomes 
