Hyatt.] 302 [April 4, 



and Present, which have tubular siphons, rounded abdomens pre- 

 cisely as in the more radical or lower species of Cymatoceras, and 

 are separable from this genus only by the absence of tran verse 

 costae at all stages. The young sutures in some Cretaceous 

 species, Naut. Dekayi, for example, have first ventral saddles, then 

 in later stages ventral lobes, and in the adults ventral saddles are 

 developed. We have, also, not yet seen a full grown shell among 

 Jurassic species, which had the annular lobes though they are some- 

 times retained until the latest adolescent stages. The abdomen is 

 sometimes flattened, as in Cenoceras, but the young even in such 

 species have gibbous, or kidney-shaped whorls, they also invariably 

 present longitudinal ridges, which in some species, may even be 

 said to be slightly subspinous. The sipon in the young is between 

 the centre and the ventral side, and in the Jurassic species retains 

 this position in the adults. In Cretaceous species it appears nearer 

 the centre. In the Tertiary species there seems to have been 

 no lobe developed on the venter at any stage, and in the recent 

 species, all three of which we have examined, there is, also, no 

 abdominal lobe at any stage. The adult lobed stage of the Juras- 

 sic progenitors is skipped in accordance with the law of concen- 

 tration of development in descendent forms, and the larval saddles 

 are perpetuated without change throughout life. The young of 

 the modern Nautili do not resemble the adults of Cymatoceras 

 or of Cenoceras, and in our opinion have only one nearly related 

 form, the ridged adults of Nephriticeras. We have to go back 

 to the later larval and adolescent stages of the transition forms of 

 the Jura in order to see that there may have been direct connec- 

 tion between the genera Cinoceras and Nautilus, and some of 

 the facts indicate that the separation of the two genera may have 

 taken place in the Trias. Thus, Naut. striatus, sp D'Orb. and in- 

 ilatus, ibid, Terr. Jurass. both have the typical form of this genus 

 and ventral sad lies. Though the siphons are above the centre, 

 and the annular lobes are retained until a late stage, as in Ceno- 

 ceras of the same period, they do not resemble them so closely in 

 their aspect as they do the lower forms of the Triassic Ceno- 



side. The funnel at this stage is, therefore, macrochoanitic, bnt not holochonnoida 1 

 as formerly supposed. The funnel of the third septum is much shorter. The micro- 

 choanitic characteristic appears to be due to the length of the chamber and not to any 

 great difference between the length of these first two funnels. 



