﻿BRITISH FOSSIL CEPHALOPODA. 25 



feature, as well as in others, the two suborders were as sharply defined when first 

 found together as at a later period. This cap has been called an "ovisac," but that 

 is a word already appropriated by zoologists for a case to contain the ova, and 

 cannot therefore be applied to a part of the embryo. There seems also to be no 

 reason why it should not be called the nucleus, as the similar part of a Gastropod 

 shell would be. The several interesting points about this nucleus in the Ammo- 

 nitoidea cannot now be touched upon, but we must confine ourselves to the 

 Nautiloidea. 



In these the nucleus presents no discontinuity of curvature, but is merely the 

 natural rounding-off of the form of the shell, or its coming to a point with a larger 

 apical angle. On the exterior surface of this nucleus in the recent Nautilus is 

 situated a narrow depression, with its long diameter in the plane of symmetry, and 

 its sides swollen and smooth (PI. II., fig. 5, a). This was first discovered by Dr. 

 Hooke at the end of the 17th century, and published in Derham's ' Philosophical 

 Experiments and Observations' in 1726. Hooke supposed it to be an air-hole into 

 the siphuncle. Barrande, with his usual closeness of observation, did not let this 

 point slip him ; but it has been made a special study by Alphseus Hyatt, 1 whose 

 observations are very careful, and his figures very close to nature (PI. II., fig. 5). 

 He calls this depression a cicatrix or scar, and considers that it marks the passage 

 through which the growing animal escaped from a nucleus similar to that of the 

 Ammonitoidea, but which was not persistent. He remarks on the difficulty of the 

 animal passing through so narrow a slit, though there is no reason why the aperture 

 should not have been larger at first and diminished by an aftergrowth. 2 There is 

 not, however, the slightest proof that there ever was any nucleus beyond what we 

 see, and every probability that there was not. The embryo Ammonite has its first 

 septum at the junction of the nucleus to the later shell, and the siphuncle com- 

 mences in the former ; so in the Nautilus the first septum is at the junction of this 

 nucleus with the normal form, and the siphuncle passes into the nucleus (PI. II., 

 fig. 4). Thus to assume an earlier nucleus involves the assumption of a later 

 development of the siphuncle, for this organ is quite cut off by the shell at the base 

 of the nucleus, where it commences. Doubtless the nucleus differs only in form 

 from that of the Ammonitoidea, and the cicatrix marks the point from which the 

 growth of the shell commenced. 



A section of the shell in the plane of symmetry taken along the cicatrix (PI. II., 

 fig. 4) shows that the siphuncle does not commence exactly opposite to it, but nearer 

 to the convex side of the shell. The surface of the cicatrix (fig. 4, a) is very 

 undulating, as though modelled on a loose membrane ; but the substance of the shell 



1 " Fossil Cephalopods of the Museum of Comp. Zoology — Embryology." — Bull, of Mus, Comp t 

 Zool, vol. iii. No. 5, 1875. 



2 See Owen, Proc. Zool. Soc. 1879. 



E 



