104 



THE QUARTERLY REVIEW OF BIOLOGY 



a hyponomic funnel is essential for respira- 

 tion; it ignores the universal principle of 

 organisms, namely: that a dominant 

 group tends to utilize all available habi- 

 tats more or less completely; and it 

 disregards the meaning of the wide 

 geographic range that a number of species 

 show. 



Narrow planospiral shells fulfil what 

 might be called the stream-line form for 

 disks. Many, such for example as a 

 large Sphenodiscus, have lines that ob- 

 viously indicate a movement through the 

 water, and how such high and narrow 

 shells could have been manipulated by a 

 crawling animal is incomprehensible. It 

 is true that there are a number of genera 

 of benthonic gastropods scattered through 

 many families and sub-orders that have 

 planospiral shells. In the planospiral 

 gastropods the shells are generally small 

 and their apertures wide — usually wider 

 than high. None approach the cephalo- 

 pod shell-form, unless it is the tiny early 

 stage of the genus Caecum, so that I think 

 it may be conceded that the normally 

 lenticular form of the coiled Cephalopoda 

 — whether nautiloids or ammonoids — is 

 indicative of a swimming habit. 



There are, however, great differences in 

 the transverse profiles of their shells, 

 which doubtless indicate different degrees 

 of resistance to slipping through the 

 water, and in a general way ammonites as 

 a whole are more compressed than nauti- 

 loids, the most compressed ammonites, 

 with "cut water" keels or venters, being, 

 in general, among the later forms. 



Among the coiled shells of both nauti- 

 loids and ammonoids the swimming 

 ability was not only conditioned by the 

 form of the shell and the resistance it 

 offered to passage through the water, 

 but also by the power of the hyponomic 

 funnel, and there is no means of obtaining 

 information on this second factor, since I 



do not regard the presence or absence of a 

 hyponomic sinus as a reliable indication. 

 This last may perhaps be used with 

 caution, but there are forms — Placenticeras , 

 for example — that I regard as active 

 swimmers, but that show no hyponomic 

 sinus. 



Transverse profiles of various forms from 

 all horizons show variations from wide 

 ellipticity to extreme compression. The 

 existing Nautilus, which has almost as 

 blunt prowed a shell as can be imagined, 

 (Plate 6, fig. i), a shell certainly wider 

 than in the majority of fossil forms, is' 

 known by observation to swim moderately 

 well, which shows how cautiously shell 

 form must be used in predicating locomo- 

 tive ability. I have shown three more 

 depressed and wider forms in the accom- 

 panying illustrations — a PolyptycMtes: 

 (Plate 6, fig. 2.) from the Lower Cretaceous,: 

 a Tropites (Plate 6, fig. 3) from the Triassic,; 

 and a Cyclolobus (Plate 6, fig. 4) from thei 

 late Paleozoic, all of which would seem: 

 to have been exceedingly poor swimmers.; 

 For comparison with these a profile of 

 Placenticeras, with what I regard as fast; 

 lines, is shown. I believe that all highly 

 compressed forms, and the ammonites: 

 show them at all later horizons — Sageceras, 

 and Pinacoceras Qayeri Hauer) from thei 

 Triassic, Oxynotkeras from the Jurassic, 

 Placenticeras and Spenodiscus from the Upper; 

 Cretaceous, etc. — were rapid swimmers.; 

 Otherwise their shell form is meaningless, 

 and it is impossible to imagine thet 

 animal as having been able to handle! 

 such high and narrow shells in any other: 

 way. 



There are shell forms, however, both 

 among Nautiloids and more commonly 

 among the Ammonoids, appearing sporad- 

 ically throughout all cephalopod history 

 from the Devonian to the Upper Cre- 

 taceous, that became secondarily adapted 

 for a benthonic mode of life. All trochoid 



