NOMENCLATURE OF STAGES OF GEOWTH AND DECLINE. 



11 



It is obvious that we cannot ac- 

 count for the nautilus-like ven- 

 tral saddle of the earlier sutures 

 of the Ammonoids, the calca- 

 reous shell of the protoconch, 

 the coecal stage, the absence of 

 the collar in the lower Goniati- 

 tinae and in the young of the 

 higher forms, the often central 

 position of the siphon in the 

 young, and many other charac- 

 ters, unless we admit a proba- 

 ble derivation of the Goniatitinre 

 from some straight microsipho- 

 nulate form of Nautiloid. It is, 

 therefore, highly probable that 

 the pteropod-like aspect of the 

 young of some Goniatitinoe may 

 be a purely homoplastic charac- 

 ter, and be meaningless so far as 

 the genesis of the group is con- 

 cerned. 



7. The next or second of the 

 nsepionic stages was represented 

 by a living chamber, which was 

 completed by the building of the 

 first septum with its attached 

 caacum, indicating the primitive 

 beginnings of a siphon. This 

 stage we styled the Csecosipho- 

 nula, and we have considered the 

 possession of a caecum to be an 

 indication of the former exist- 

 ence of an ancestor having a 

 central series of cascal pouches. 

 These may have had functional 

 communication in some forms by 

 means of an endosiphon, as in 

 the Endoceratidos, and in others, 

 either belonging to this family or 

 to a more primitive group, they 

 may have been closed creca. 



8. The next ntepionic stage 

 was ended when the second sep- 

 tum was built in the modern 



lio 

 d gh 



Fig. 9, 10. Views from the side and below of the plug which the 

 animal of Orth. truncation, Barr., habitually built on the exterior 

 of the broken or truncated end of its shell. The last suture 

 is shown in Fig. 9, and the internal shadowy markings are 

 apparent in both figures at a, g. These, however, in Fig. 9, 

 are too far removed from the exterior. When the outer layer 

 of the plug is penetrated, they are seen to be a part of its 

 structure. The side view is also defective in the drawing of 

 the pseudo sipiion (d). There should be three distinct steps 

 indicating three layers. The external crenulated stria; of the 

 plug appear at h. Loc. Bohemia, Coll. British Museum. 



Fig. 11, 12. Views of the same from the side and below, to show 

 the external markings of the plug (h), which contrast strongly 

 with the perfectly smooth shell above the septum of trunca- 

 tion and internal stria; (1) which appear when the outer layer 

 is fractured No septa ever occur in the plugs. These figures 

 are introduced in order to meet M. Barrande's objections (Syst. 

 Syl, PI. 488), that the examples of what we have called the 

 protoconch and apex of the true conch were in reality plugs 

 similar to those of Orth. truncation. There is no need of mak- 

 ing any remarks; if our figures are correct, we are right in 

 our statements. It may, however, be well in this connection 

 to say that M. Barrande has done us the honor to make use of 

 a number of our figures, including in part the above. 



Fig. 13-15. Views of the cicatrix and apex of Orth. unguis, Phill., 

 after the shedding or removal of the protoconch as it usually 

 occurs, leaving the cicatrix uninjured. Fig. 13 shows the area 

 of the cicatrix much enlarged ; b, conch or apex forming a 

 smooth shoulder; and c, depressed surface of- the cicatrix. 

 Fig. 14, view less magnified of apex; Fig. 15, section of same. 

 Loc. Dublin, Coll. British Museum. 



Fig. 16. Apex of Orth. unguis, Phill., natural size, with first three 

 sutures. 



Fig. 17. Apex of same species after the probably violent removal 

 of the protoconch, showing the fractured shell (b), and the 

 unusual aspect of the cicatrix. This and F'ig. 16 are types. 

 Loc. Yorkshire, Coll. British Museum. 



Fig. 18. Front view of Fig. 19. The broken line (k) is hypotheti- 

 cal. It indicates the possible outline and position of the caecum, 

 supposing the oval area in the centre of Fig. 19 to have repre- 

 sented that organ. 



Fig. 19. Apex of Orth. politum from below The protoconch has also 

 been violently removed, and the opening plugged, apparently 

 from within. The dark spot on the right seemed to be a rup- 

 ture in the external surface. The oval shade in the centre indi- 

 cated an internal structure, which may have been the csecura in 

 the first air-chamber. Loc. St. Ca'ssian, Coll. British Museum. 



