MUSEUM OF COMPARATIVE ZOOLOGY. 89 



inflection, which is alone retained in the adult, should be regarded as a 

 remnant of the more complete lobe of the young. In the Aganites 

 group, the lobed and its accompanying sutural inflection are present 

 whenever the siphon funnel is not too close to the dorsal side. When 

 this occurs, as in Nautilus zic-zac, the lobes of the septa are not 

 developed, but the sutural inflection appeared to be distinctly marked. 

 "Whether this was really persistent, or whether the depressions I saw 

 were due to the violent removal of a portion of the funnel, was not 

 satisfactorily determined. The dorsal lobe of Nautilus Pompilius im- 

 pinges against the internal portion of the upper end of the cicatrix, 

 just as the siphonal ccecum does against that of the lower end, and is 

 apparently the result of the passage of some organ or part from the 

 ovisac into the shell, but the entire outline of the first septum in the 

 young of Nautilus atratus, N. Koninckii, is conclusive against such a 

 supposition.* It is evidently only another of the same kind as those 

 characteristics already cited, which are developed earlier in later 

 species of .the same genetic series, according to the law of acceler- 

 ation of development. When compared with the small area of the 

 first septum of Nautilus Pompilius, it is seen to be a large and very 

 well marked lobe. The size, however, does not increase propor- 

 tionally with the growth of the shell and the area of the septa, and 

 thus in the adult it becomes comparatively insignificant. During the 

 younger stages, also, it modifies not only the sutures, but forms a 

 decided conical depression in the septa themselves, so that the latter 

 bend posteriorly at their junctions with the dorsal side of the shell. 

 The lobe in the suture, however, is formed higher up, just under the 

 cone of the next older septum. This is due to the great extension of 

 the septa of Nautilus on the dorsal side, which reach so far that they 

 are overlapping or imbricated. This cone disappears on the first 

 quarter of the third whorl, and the septa, instead of bending posteriorly 

 into a lobe, are simply rectangular with a minute depression on the 

 surface. This, however, speedily disappears, and a slight ridge, evidently 

 marking the trace of some organ or part, makes its appearance. This 

 is found first bisecting the sutural lobe, and seems to extend con- 

 tinuously underneath the septa built upon the inner surface of the 

 dorsal side of the shell. The septa, after the appearance of the ritlge, 

 lose their rectangularity, and become evenly concave, the narrow, 



* Piute IV, Figs. 5-9. 



