9§ 



THE QUARTERLY REVIEW OF BIOLOGY 



backward was awkward is wholly anthro- 

 pomorphic — it is to us, but is decidedly 

 not for a cephalopod. The other idea 

 that they would run the risk of fracturing 

 the apex of the shell is altogether fanciful 

 — they did not find their optimum condi- 

 tions of life on a stern and rock-bound 

 coast, and in any event nature usually 

 provides against accidents in its tendency 

 to overpopulation. (Pathological rostra 

 of the Mesozoic belemnites are found 

 fossil.) 



Many students can not dismiss the idea 

 from their minds that the cephalopod 

 shells were too heavy and too unwieldy 

 to have been propelled by the relatively 

 simple method in vogue among these 

 animals. This rests almost entirely upon 

 the handling of museum specimens in 

 which the chambers are filled with sedi- 

 ments, lithified, or calcified, or impreg- 

 nated, or replaced with marcasite, etc. 

 In the vast majority of forms throughout 

 their whole geologic history, the shells 

 were thin and in life were relatively light 

 and easily propelled, and the specific 

 gravity of the whole animal was close to 

 that of the medium — so close, I imagine, 

 that when the animal was expanded to 

 its maximum extent it would float or rise 

 in the water, and when withdrawn within 

 its living chamber it would sink. 



A belief in the swimming habits of the 

 majority of orthoconic forms is also a 

 reason for regarding them as having had 

 "arms" like a squid rather than tentacles 

 like a Nautilus, since when swimming 

 these would trail behind, as do the ex- 

 tended tentacles in the Nautilus, and 

 would furnish the necessary rudder to 

 guide their passage through the water 

 more effectively than tentacles. Ob- 

 viously not all orthoconic forms were 

 swimmers. I would expect such swim- 

 ming forms to show some compression in 

 the cross profile of the shell, and this is 



perhaps true in a majority of cases. 

 Crawling or grovelling forms should show 

 some depression in the cross profile of the 

 shell, and this can be seen in a number of 

 forms. 



The extreme example of depressed shell 

 form, undoubtedly indicating a sedentary 

 bottom-dwelling animal, which had prob- 

 ably lost the swimming habit and could 

 move only by dragging its shell by the use 

 of its arms, is the genus Gonioceras (Hall, 

 1847). This includes five or six Ordo- 

 vician species found in Ontario, New 

 York, Illinois, Wisconsin, Minnesota, 

 and northern China. The shell is several 

 times as wide as it is high, with lateral, 

 flanges; the septa are closely spaced and 

 sinuous; and the siphuncle is large and 1 

 nummuloidal. At maturity the aperture 

 is greatly constricted. Since the muscular 

 attachment of the animal to its shell is not 

 especially firm throughout the Cephalo- 

 poda, this constriction of the aperture in 

 Gonioceras would serve to prevent the 

 rupture of the muscular attachment when 

 the shell was pulled along over the 

 bottom, and the angularity of the septa' 

 would also serve to give the animal a 

 firmer hold on its shell. Hyatt (1884 and 

 1900) made some wild guesses regarding 

 the affinity of this curious form; we are 

 not, however, at present concerned with 

 its taxonomic position, but only with its 

 obvious adaptation to a benthonic exist-, 

 ence; from this point of view it is com- 

 pletely adapted, and ceases to be curious — 

 the large nummuloidal siphuncle may be 

 regarded as ballast, helpful in maintaining 

 an even keel (Plate z, fig. 17). 



I have already mentioned the habit of some of the 

 earlier orthocones of using the siphuncle in this way, 

 or of filling the earlier chambers to a greater or less 

 degree with organic deposits to counteract their 

 buoyancy. Large and often elaborately ornamented 

 siphuncles occur in a number of families of nautiloids, 

 as for example in the families Endoceratidae, Pilo- 





