604 EECORD OF CURRENT RESEARCHES RELATING TO 



older slicll, is probably an example of a form whicli did early separate 

 itself from the common stock. 



The next point discussed is one of considerable difficulty ; it is 

 the relations of the Octopoda to the two other groups. The high and 

 exceedingly peculiar organization of the Octopoda seems almost 

 certainly to point to their isolation for a long period of time, or, in 

 other words, to their derivation from some other group than the 

 Myopsida. When we examine the Loligopsis-group of the CEgopsida, 

 we find that they alone among the Decapoda present some of the 

 peculiarities of the Octopod organization — the absence, namely, of a 

 valve to the funnel, the presence of a well-developed spleen, and the 

 rudimentary apparatus for closing the mantle. As we may suppose 

 that the primitive Dibranchiata possessed certain arrangements, such 

 as the fusion of the supra-pharyngeal ganglion with the cerebrum, &c., 

 which are now only seen in the Nautilus and in the Octopoda — it 

 seems allowable to suppose that these creatures were separated into 

 two sets, one of which diverged into the Ommastrephida, and the 

 other into the common stem-form of the Loligopsida and Octopoda. 

 This view has its objections. 



To sum uj) : it seems clear that the Dibranchiate Cephalopoda 

 may be divided into three distinct phyla. The oldest are the 

 CEgopsida ; the two others — the Myopsida and the Octopoda — have a 

 closer genealogical relation to one another. The Q^]gopsida may be 

 divided into two groups — the Ommastrephida and the Loligopsida. 

 It is probable that the (Egopsid forms passed through a Belemnite 

 stage to the Sepias, and that the Decapoda with horny shells divei'ged 

 as independent branches at different times. The Octopoda, or most 

 differentiated forms, have evident points of relationship to the Loli- 

 gopsida ; this derivation may not have been altogether simple. 



The parallelism in mode of development of the groups is very 

 striking ; it is best shown in the tendency to reduce and lose the 

 shell. In the oldest phylum we find the phragmocone, or a simple 

 horny shell; in the Myopsida we have Sepiola and Bossia, with a 

 shell only half the length of the animal. Cirrhoteuthis, an old 

 Octopod, has a distinct internal shell ; but in the more developed 

 forms, not only is the shell lost, but is typically so. 



The long essay ends with a discussion on the general bearing of 

 the facts detailed on the doctrine of descent. 



Aptychi of Ammonites. — In the dwelling-chamber of Ammonites 

 is sometimes found a remarkable body — the aptychus — resembling a 

 bivalve shell widely opened. Very various opinions have been held 

 about these bodies ; some considering them to be the opercula of the 

 Ammonites, whilst Pictet avowed that it was difficult to j)rouounce 

 upon their true affinities.* 



Mr. C. Moore has already challenged the correctness of the 

 operculum view, and in a fui-ther paper f he shows as the result of 



* For a summary of the views of English and foreign observers, with references, 

 see " The Lias Ammonites," T. Wright, ' Palajoutographical Soc./ xsxiv. (1880) 

 p. 182. 



t ' Kept. Brit. Assoc,,' lS7t), p. 311. 



