i; EINAR LÖNNBERG: NOTES ON SPIRULA RETICULATA OWEN 



11.; 



they arc not useful and the shell is simplified in every respect. If w< suppose that 

 the shell of an Ammonite was transformed according to such rules, it would then be 

 very similar to the shell of Spintla. 1 therefore believe, that we have to look for the 

 ancestors of Spirula among the Ammonites. There is nothing known about th< 

 anatomy of these animals. We have only to judge from what the shell in 

 It has already been said that the initial chamber, the prosipho and the siph 

 a certain degree resemble the corresponding parts of a Spirula and we ha 

 pointed out that the structure of the shell is dependent on its position and function, 

 so that in these respects the similarity of the shells of Ammonoidea and Nautibidea 

 is an analogy, that is the more striking as it concerns homologous organs. So far 

 as knowledge at present extends, there seems to be nothing that prohibits us from 

 regarding the Ammonoidea as ancestors of Spirula and tetrabranchiates or an 

 of tetrabranchiates. Moreover, we need some ancestors of the Belemnites of a more 

 primitive type than Belemnoteuthis. For in that form the shell is already in n 

 grade development. The ancestors of the Belemnites must have had, we may 

 conclude, a straight shell, with the animal living in the last chamber; thus, some form 

 resembling in its shape a Bacirilcs or a Bacvlina. 



Several of the known Ammonites are very similar to Spirula at least as lat- 

 as shape is concerned; take, for instance, Mimoceras. Yet this form is counted among 

 the Goniatitidcc with a dorsal sipho. In most Ammonoidea with the exception of 

 Clymcniida: the sipho is found on the convex or dorsal wall of the shell. Hut the 

 situation of the sipho is not quite constant. In Tropilidee it is first ventral (on the 

 concave side), becomes later central, and finally dorsal (on the convex side). Zittel 

 says about this inconstancy: "Bei den meisten jüngeren Ammonitcn hat der Sipho 

 zuerst centrale und erst später randstündige Lage". It is therefore easy to under- 

 stand that the sipho may have changed places during the phylogenetic development 

 of Spirula. The complex sutures of the septa of some Ammonoidea do not present 

 any difficulty for deriving Spirula from an Ammonite, because the first sutures ar< 

 complex, moreover, as the shell becomes more and more enveloped in the mantle it 

 remains in a simpler and more juvenile stage of development. Of the outer orna- 

 mentation of the shell we still find some traces in Spirula in the form of small tu- 

 bercles or elevations on the surface. This fact also points to the Spirula shell 

 having once been external, because internal shells, as a rule, have a perfectly smooth, 

 unornamented surface. 



The anatomy of Spirula as Pelseneer remarks (1. c. p. 31) has some 1 

 in common with the OEgopsids and these may all be of an ancient character as some 



Ftstskri/t fur Lilljeburg. 





