312 On the Homologies of the Cephalopoda. 



ately beneath them, is rather difficult to say, and is one point 

 which would be settled by a knowledge of the Nautilus's 

 development. On the one hand, as was originally suggested 

 by Van der Hoeven, and has been expounded recently by 

 Owen"*, the hood in the extinct allies of the Nautilus had the 

 power of secreting calcareous or horny matter known as the 

 aptychus', and this leads us to the shell of the Argonaut, secreted 

 by the anterior pair of arms, which would thus be homologous 

 with the aptychus if the hood were a modified pair of arms. 

 On the other hand, if we carefully examine the upper surface 

 of a Sepia and other Decapods in front of the calcareous "bone" 

 and just behind the eyes, we shall find two hardened plates 

 (called neck-plates by Keferstein), whose shape and orna- 

 ments are so similar to those of the aptychus as to make us 

 almost certain of their homology ; and these, therefore, must 

 represent the hood of Nautilus, with whose position they agree. 

 Yet we do not know that these two plates are in any way 

 connected with the arms, either in the adult or during deve- 

 lopment ; but they seem to belong to the anterior part of the 

 epipodial ring. Either homology is so interesting that one 

 would wish to find some "way of adopting them both. 



Finally, is the bone of the Sepia homologous with the shell 

 of the Nautilus ? Not exactly, I think. The homologues of 

 the latter may be seen in the shell of the Spirula and the 

 phragmocone of a Belemnite ; but any representative in the 

 Sepia must be sought in its mucro, and not in the mass of the 

 bone. This opinion (for at present it is little more than an 

 opinion) seems to gain weight by a consideration of the fossil 

 genus Ascoceras. This occurs in the Upper Silurian strata, a 

 very probable date for the near approach of the Dibranchiates. 

 In it we find two sets of septa : the one set are at the base of 

 the shell of the ordinary kind pierced by a siphuncle ; the 

 other set are in the body-chamber. They lie on one side 

 obliquely ; they run into one another in their curves ,' and they 

 are penetrated by no siphuncle. In other words, the shell 

 presents us with the characters of the Nautilus-sheli at its 

 base, and with those of the Sepia-bone above ; and from it we 

 may perhaps perceive the true relations of these two structures. 

 Further details on this genus cannot now be entered upon ; 

 but they will be given in my forthcoming ' Monograph of the 

 Fossil Cephalopoda of Great Britain.' 



Proc. Zool. Soc, Jan. 1879. 



