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 Glyphioceras. The septa of this stage 

 are shown on Plate IV, Fig. i, 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 Anarcestes, 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 10, 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 i 



