MUSEUM OF COMPARATIVE ZOOLOGY. 67 



Saemann, in Dunker and Meyer's Palajontographica,* has shown 

 that Lytoceras Jimbriatus had a distinct globular ovisac, and figured the 

 young of this species in comparison with the pointed young of Nautilus 

 atratus. 



D'Orbigny describes the aspect of the apex of the whorl in Nautilus 

 as an obtuse cone, and in Nautilus lineatus of the Jura, writes of this 

 cone as resembling a Patella. Barrande, in criticising this view, re- 

 marks very justly, that the cone must have been at one time the living 

 chamber of the animal, and as this must have extended for some dis- 

 tance, there are no grounds for comparing it with the flattened cone of 

 Patella. 



When observed at the bottom of the umbilicus, the ovisac of the 

 Ammonites appears as an oval body, generally more or less denuded 

 of the shell, which breaks away with the matrix.f When the whorls 

 which encompass it are removed, the ovisac is seen to be much larger 

 than these exposed lateral areas, which are merely extreme portions 

 of the narrow sides of the embryonic shell or ovisac* The entire 

 form, when seen from the side, is that of a very broad symmetrical 

 oval, flattened considerably on the abdomen, § and lenticular when 

 viewed from the abdominal or dorsal sides. If 



These outlines may vary considerably in the same species. In some 

 specimens of Deroceras planicosta the abdomen is flatter than in others, 

 and often depressed as it nears the sides, giving the latter a remarkably 

 bulging aspect.** 



Among the Arietidge, the species examined, Arnioceras semicosta- 

 tum,ft and Asteroceras obtusum \\ presented no considerable variation, 

 except such as would naturally result from difference of size in the 

 species. 



Even in the abnormal forms, Scaphites and Crioceras, the ovisac fills 

 the fundus of the umbilicus, or, in other words, is closely enveloped by 

 the first whorl. Whether the young of the still more uncoiled genera, 



* Vol. Ill, p. 158, pi. 19, Figs. B, C. 



f Plate I, Figs. 7, 8. Plate II, Fig. 9. 



% Plate I, Fig. 6, A. Plate II, Fig. 7. 



§ Plate I, Fig. 4. 



1F Plate I, Figs. 1, 2. 



** Compare Fig. 2 with 5, and Fig. 6 with 1 on Plate I. 



ft Plate II, Figs. 8, 9. 



%X Plate II, Fig. 11. 



