4 o6 SCIENCE PROGRESS 



by the Palaeozoic families of the Lituitidcv and Trochoceratidce ; 

 here also it appears to be a mark of senility in the race. In 

 the Ordovician and Silurian genus Lituites (in its present 

 restricted sense) the shell is at first coiled in four whorls and 

 then is unrolled in a very long straight portion, gradually 

 widening in diameter, with an aperture partially contracted 

 by lobes. The straight piece is bent slightly inwards near its 

 commencement, just as in Macroscaphites (fig. 7). Noetling 1 

 considers that the development of Lituites lituus shows indica- 

 tions of the descent of this genus from Nautilus-like ancestors. 

 The allied genus TrocJwceras, which ranged from the Cambrian 

 to the Devonian, now contains several well-known species 

 formerly ascribed to Lituites — e.g. T. cornu-arietis, T. arietiuum, 

 T. gigauteuiu, and T. couvolvans ; but the whorls rarely exceed 

 two in number, and the unrolling has not proceeded so far 

 as in Lituites ; it only shows an incipient unrolling in the 

 last whorl. 



With regard to the Turrilite form of the unrolled Ammon- 

 ites, Quenstedt, Oppel, and in later years Hyatt, 2 have shown 

 that this form, for instance, often occurs transitorily in the 

 young shells of several species of Ammonites, and becomes 

 completely suppressed during the later period of growth, 

 when the whorls develop normally in the same plane. The 

 persistence of a Turrilite form in the adult is, therefore, clearly 

 retrogressive and reversionary, and may be regarded as a more 

 highly developed phase of a juvenile variation. Even all this 

 bizarre variation of form did not avail to perpetuate the race 

 of Ammonites, and not a single species survived the Cretaceous 

 period. It must be remembered that the Ammonites had diverged 

 in habit from the swimming Nautiloids and had adopted littoral 

 habits, crawling along the bottom w r ith their shells above them, 

 and very rarely swimming. The presence of a rostrum, in fact, 

 points to the disappearance of the hyponome, or swimming 

 organ of Nautilus. The strikingly progressive complication of 

 the suture-lines of the Ammonites was correlated with their 

 usefulness in assisting the balance of the shell above the 

 extended parts of the animal whilst crawling along the sea 



1 " Ueber Lituites lituus" ZeitscJn-. deutsch. geol. Gesell., 1882, Bd. xxxiv. Heft 1, 

 pp. 156-95, Taf. x. xi. 



2 " Embryology of Fossil Cephalopods," Bull. Mus. Coynftarat. Zoology, 

 Harvard Coll. iii. No. 5, p. 72. 



