THE QUARTERLY REVIEW OF BIOLOGY 



lateral sinuses, and fairly well developed 

 dorso-lateral crests. The living chamber 

 was extraordinarily long, and the conse- 

 quent weight of the animal must have 

 caused the shell to assume an almost 

 vertical attitude in the water. The adult 

 Lituites must have drifted, head down, 

 picking up its food on or near the bottom. 

 If the hyponomic funnel remained func- 

 tional, as the presence of a hyponomic 

 sinus would seem to indicate, it would 

 serve to propel the animal upward, which 

 would then sink slowly. A very slight 

 action of the funnel would serve to keep 

 the animal at any favorable horizon where 

 food happened to be abundant. 



Among the ammonoids floating adapta- 

 tions are expressed in various ways. 

 Least modified are various genera of the 

 family Stephanoceratidae, such as Norman- 

 nites Munier-Chalmas of the middle 

 Jurassic, in which the aperture is so 

 contracted (Plate 5, fig. 5) that it is 

 difficult to visualize a functional hypo- 

 nomic funnel, although this is not 

 impossible. Another involutely coiled 

 small form which seems to have been 

 pelagic is the genus Morphoceras Douville 

 (Plate z, fig. 9) of the Jurassic and 

 Cretaceous, in which the adult aperture 

 was almost entirely closed and swimming 

 would seem to have been precluded 

 (Plate 5, fig. 4). I have pictured it as 

 planktonic. 



Modified in a different way are those 

 forms which were tightly coiled in their 

 youth, but which have the very long 

 adult living chamber suspended, and 

 frequently recurved near the aperture. A 

 good example of this adaptation is fur- 

 nished by the Lower Cretaceous genus 

 Macroscaphites Meek (Plate ±, fig. 10), 

 which, like the Ordovician nautiloid 

 Lituites, must have been a swimmer in its 

 youth. At maturity the body chamber 

 hangs down for a distance about equal to 



the diameter of the coiled disk and 

 finally curves upward at its lower end. 

 I consider Macroscaphites to have normally 

 drifted, foraging over the bottom, the 

 position of the aperture raising the head 

 above the bottom in an advantageous 

 manner. Possibly the specific gravity 

 was so adjusted that the shell would sink 

 when the animal was withdrawn within 

 the living chamber, and rise when it was 

 extended; or when extended and foraging 

 the funnel, by its action, might either 

 reinforce the induced buoyancy or tilt . 

 the hook-shaped distal end of the shell 

 so that the animal had no difficulty in 

 seizing its prey on the bottom. 



The genus Hamites Parkinson, of the 

 Cretaceous, functioned exactly as did 

 Macroscaphites, only in the former genus 

 the early shell was not tightly coiled but 

 was a gyrocone. All of the members of 

 the sub-family Scaphitinae, as exemplified 

 by the genus Scaphites Parkinson, a world- 

 wide Cretaceous type, were more or less 

 floaters. Indeed it is difficult to see how 

 the adult of such a species as the Cenoman- 

 ian Scaphites aequalis Sowerby could have 

 swum at all. On the other hand it is 

 readily conceivable that a form with the 

 aperture oriented as in Scaphites spiniger 

 Schluter — a German Upper Cretaceous 

 species — might well have been able to 

 swim. 



Still another type is illustrated by such 

 genera as Spiroceras Quenstedt of the 

 Jurassic (Plate z, fig. zi), or Crioceras 

 Leveille and Ancyloceras d'Orbigny, of the 

 Cretaceous. In these the shell is a 

 gyrocone, and such a type might well be 

 a form leading in the direction of Hamites 

 (supra). Another form that appears to 

 me to indicate a floating existence is such 

 a species of Heteroceras as Heteroceras 

 reussianum d'Orbigny (Plate z, fig. zz) 

 from the Upper Cretaceous of Germany. 

 This was relatively a small form with a 





