MUSEUM OF COMPARATIVE ZOOLOGY. 99 



anterior extension is apparent.* In all cases this anterior deflection of 

 the septum forms an open collar surrounding the sheath. The lining 

 membrane of the chambers, of course, surrounds it as well as the sheath, 

 but otherwise it cannot be considered as connected with the sheath. The 

 difference between Ammonites and Goniatites consists, then, so far as 

 the adult siphon is concerned, in the possession by the former of a more 

 comjdetely rounded siphonal coccum and the siphonal collar. Besides 

 the earlier development and different form of the siphonal ccecum in the 

 Goniatites and Ammonites, as compared with Nautilus, we find extensive 

 differences in the formation of the siphonal funnel. This, instead of 

 lining the interior of the co3cum by its extension from the second sep- 

 tum, as in Nautilus, only reaches the mouth of the opening through the 

 first septum, and in the succeeding septa, instead of reducing the length 

 gradually, the funnel becomes at once, in the third septum, very short 

 and distinct from the sheath. They all three, Nautilus, Ammonites, 

 and Goniatites, agree, however, in having, during the earlier stages of 

 development, a siphon formed by a ccecal prolongation of the first sep- 

 tum. The large size of this ccecum, as compared with the area of the 

 first and second septa, is also an important fact in this connection. 



If there is any truth in the application of embryology to the solution 

 of the problem of evolution, or even of the relative rank of forms, it is 

 evident that in either sense the true prototype of the Cephalopods 

 must have these characteristics, namely, a large siphon, composed 

 of circular prolongations of the septa, or siphonal funnels closed at 

 their posterior ends. Consulting the development of the simpler Nau- 

 tilus, we see also that these siphonal funnels should set one into another, 

 like a pile of cups, or cones, and the septa be concave and very 

 shallow, as in the Jurassic and Carboniferous species. The conditions 

 are partially fulfilled by the genus Endoceras, whose septa are shallow 

 and concave, and the siphon consists of a series of cones, placed one 

 within another, and closed at the posterior extremity. These cones are 

 not strict siphonal funnels ; the funnel portion really only extends from 

 the opening of one septum to that of the next, and the cones, which 

 are evidently the homologues of the sheath of Nautilus, are built 

 against the continuous wall thus formed, as partitions in the siphon 

 itself. These are the characteristics of the adult only, and it can be 

 reasonably anticipated that the young of Endoceras would exhibit a 



* Plate II, Fig. 1. 



