﻿619 



Plate II. 



Figs. 1-6. Mimoceras (Goniatites) compression, after Branco, Paleontogra- 

 phica, xxvii, PL viii, much enlarged, to show the loose mode of coiling, absence 

 of impressed zone, primitive nautiloid sutures of the young until a late stage, and 

 protoconch joining the conch without any constriction on the sides or abdomen, 

 but with a slight constriction on the dorsum, showing tendency to coil, these 

 being typical ammonitoid characteristics. Fig. 6, I and 2 and x represent the 

 first, second, and older septum, the septa of the later stages are shown with ven- 

 tral siphonal lobe in Fig. 5. Fig. 20 is the same species after Sandberger. 



Figs. 7 and 8. Mimoceras (Goniatites) ambigena, after Barrande, Systeme 

 Silurien, PI. iii, Fig. 22, and PL xii, Fig. 7, showing the absence of an impressed 

 zone at a late stage, probably the ephebic stage of development 



Figs. 9— 1 1. Agoniatites (Goniatites) fecundus, dSttx Barrande {ibid.'), PL x, 

 Figs. 13-15, showing the absence of the impressed zone until a late stage; Fig. 

 1 1 is nearly natural size, and this shows, when compared with others of Bar- 

 rande's figures of this species, which have the impressed zone in adults (Bar- 

 rande, PL vii, Figs. 10 and n), that this characteristic comes at a much earlier 

 stage in some specimens, and is originated by close coiling, as it is in Agoniatites 

 Vanuxemi, but at an earlier stage than in A. fecundus. 



Figs. 12 and 13. Agoniatites (?) (Goniatites) oebriseptus, after Barrande 

 (ibid.), PL vii, Figs. 1 and 2, reduced one-third, showing how similar to some 

 nautiloids the adults of this genus may be, both in form and sutures,* with the 

 exception of the ventral siphonal lobe, which alone enables one to place them 

 among Ammonoids. 



Figs. 14 and 15. Agoniatites fecundus, after Barrande (ibid.), PL xi, Figs. 2 

 and 4, enlarged to show the variation of the coiling of two varieties in the pro- 

 toconchial and nepionic stages. These are both included under one name by 

 Barrande. 



Fig. 16. -Anarcestes (Goniatites) lateseptatus, after Branco, op. cit., xxvii, 

 PL vi, enlarged, showing the immature ventral sutures of this with the primitive 

 undivided siphonal lobe beginning in the second septum of the metanepionic 

 substage. This continues substantially the same in aspect throughout life in the 

 genera Mimoceras, Anarcestes, Agoniatites and Pinnacites, all Silurian forms of 

 Goniatitinse. 



Figs. 17-19. Gephuroceras (Goniatites) serratum, after Branco, op. cit., xxvii, 

 PL vi, enlarged, showing sutures of Devonian Goniatitinae with accelerated devel- 

 opment. The mimoceran undivided siphonal lobe is here shown in the metane- 

 pionic substage, 2-4 septa, but in the paranepionic, where the coil is 3 mm. in 

 diameter, the siphonal saddle arises in the centre of this lobe as a new character 

 acquired in the later stages of growth. 



Figs. 20-25. These give the protoconch and young of various species of 

 Goniatitinse of the Devonian after Sandberger, Jahrbuch d. Nass. Verein, 1 85 I, 



* Mimoceras (Goniatites) lituus is a still larger form, also figured by Barrande, which has 

 no impressed zone and is very similar to many nautiloids which have similar compressed 

 elliptical whorl and subventran siphuucles. His figure is reproduced here in reduced 

 outline, Figs. 40, 41, PI. viii. 



