9 6 



THE QUARTERLY REVIEW OF BIOLOGY 





compactly coiled shell," but as Dunbar 

 has pointed out, this explanation, like so 

 many of the natural selection sort of 

 explanations, fails to explain why, or to 

 take into account the intermediate curved 

 (cyrtocone) and incipiently coiled (gyro- 

 cone) stages, and this same author, 

 adopting Buckman's idea (1919), has 

 elaborated the true reason, namely: that 

 progressive coiling is an adjustment to 

 the buoyancy of the empty chambers of 

 the shell which induced it. 



The older naturalists, commencing I 

 believe with Buckland in 1835, believed 

 that the early chambers of the shell 

 served for hydrostatic purposes, a belief 

 due in the first instance to one of the few 

 mistakes in the account of the anatomy 

 of Nautilus given by Owen, who described 

 the axial canal of the siphon as forming 

 a communicating conduit between the 

 empty chambers and the mantle cavity. 

 When the animal wished to descend, these 

 chambers were filled with water; when it 

 wished to rise they were emptied — 

 exactly on the operative principle of a 

 submarine. This idea is perpetuated in 

 the last edition of Eastman's Zittel (1913, 

 p. 589), although Dean's (1901) observa- 

 tions on the identical buoyancy in the 

 living and dead Nautilus and Willey's 

 (190Z) demonstration that the siphon 

 does not communicate with the mantle 

 cavity in Nautilus should set at rest the 

 hydrostatic myth. The empty chambers 

 in Nautilus are buoyant once for all, and 

 what is true in this respect for Nautilus 

 should apply to all of the extinct camer- 

 ated forms that did not fill the early 

 chambers with organic deposits. 



This buoyancy of the unfilled earlier 

 chambers is the keynote to the observed 

 changes in shell form, in both phylogeny 

 and ontogeny, and the merit of first 

 applying it in any detail belongs to 

 Dunbar (1914). 



With the formation of complete septa 

 shutting off the apical chambers in the 

 developing orthocones, their buoyancy 

 in those forms that did not weight this 

 end by organic deposits filling the cham- 

 bers or elaborate deposits about the 

 enlarged siphuncle would tend to tilt the 

 shell forward. Such a tilting would be 

 a handicap in either a nectonic or ben- 

 thonic animal. Whether the resulting 

 tension on the ventral side would cause a 

 more rapid growth of the ventral shell 

 margin, or whether normal secretion of the 

 animal in so orienting its body as to keep 

 its mouth out of the mud of the bottom 

 or to remain horizontal in the water would 

 be the predominant factor cannot be de- 

 cided. In any event, the more rapid 

 growth on the ventral side would result 

 in an arcuate shell — the cyrtocone (Plate zj 

 fig. 5), and the more the curvature, thd 

 more the forward migration of the center 

 of gravity would be retarded. 



This finally resulted in the gyrocone 

 type of shell (Plate z, fig. 6). The pro-^ 

 gressive coiling would not stop with this 

 type since, as the shell is an expanding 

 cone, the newest chambers immediately 

 behind the living chamber, with their 

 exterior position and much increased 

 volume, would throughout adolescence 

 tend to tilt the animal forward, and this 

 would eventually result in the coiled type 

 of shell known as an ophiocone (Plate 

 z, fig. 7). Complete equilibrium with the 

 animal in a horizontal position most 

 effective for swimming is only attained 

 when the later chambers entirely invest 

 the earlier whorls as in the existing 

 Nautilus pompilius, a completely involute 

 form. Or an ophiocone may attain the 

 same stability if there are many volutions 

 to the shell, which serves to explain the 

 persistency of the latter type. 



The foregoing simplified series of stages, 

 illustrated by figures 1-8 of Plate z, is 



