1883.] 259 [Hyatt. 



degradation of the septa to a mere succession of calcareous 

 layers. Namely, the shell having become internal and these parts 

 being useless they gradually disappeared. They were first 

 degraded, and then lost out of the roll of hereditary char- 

 acters, the shell itself following in the same train, and disappear- 

 ing at last in the higher and more specialized Octopods. We 

 thus have Gonioceras in the Silurian, Paleoteuthis a true Sepioid, 

 Devonian ; Loligidae or Teuthidae, Jurassic ; Octopods recent. 



To clinch this evidence we can refer to the work of Lankester, 

 " Development of the Pond Snail," in which he shows that the 

 pen sac is not an enlargement of the shell gland, but must have 

 been derived from a secondary sac formed by some extension of 

 the mantle, which inclosed the shell and became a permanent 

 addition to the organization, and we differ from this author only 

 in imagining this inclosure as due to the extension of the dorsal 

 flap of the mantle, as in Nautilus, instead of to two flaps of the 

 mantle as in Aplysia. 



We regard these two orders as distinct from each other but as 

 forming a division together, which we can designate as a sub-class 

 under the name of Dibranchiata equivalent to Professor Owens 

 order of the same name. It is, however, not yet clear that 

 they arose from the same type among the straight cones, 

 though that they both came from some straight Orthoceras seems 

 to be indicated by all the evidence now in our possession. 



If now we return to the Nautiloidea and Ammonoidea we find 

 these two orders to be unitable as having external shells a com- 

 mon form of embryo and chambered shell, the chambers pierced 

 by a siphon ; that they possess similar structures, similar imbri- 

 cated layers in the shell, and similar external deposits ; that 

 they exhibit parallel series of forms in the independent reproduc- 

 tion of the nautilian shells out of straight cones, and of the 

 parallel modifications described above among the Goniatitinae. 

 There is, therefore, every probability that they have been pro- 

 perly united by Prof. Richard Owen as Tetrabranchs, We, 

 however, prefer to consider this a subclass with two orders 

 according to the classification proposed by Prof. Louis Agassiz. 



We have, after much observation, found that genetic affinities 

 on a large scale are best exhibited by the siphon, particularly by 

 the funnels of the septa, which are more invariable than any 

 other parts of the shell. 



