Miss Agnes Crane — Recent and Fossil CepJialopoda. 491 



and the circulatory and respiratory apparatus is less highly developed 

 than in the succeeding dibranchiate forms. The digestive system 

 is, however, similar, consisting of a crop and a gizzard, resembling 

 that of the common fowl. The curious fossil plates called AptycM, 

 now known as the opercula, or defensive covering to the aperture of 

 the body-chamber of the shell, of the extinct family of Ammonites, 

 were at one time considered to be the fossil gizzards of Cephalopods. 

 They were also described as bivalve shells, but the discovery of an 

 Ammonite in which they were revealed in situ, filling up the opening 

 of the shell, sufficiently indicated their true nature.^ The Nautilus 

 carries its shell snail-fashion when crawling, by means of its many 

 feet, head-downwards, at the bottom of the sea. The specimen kept 

 alive for a few hours on board the Challenger, propelled itself back- 

 wards and forwards by the expulsion of water from the funnel, after 

 the fashion of the Octopods, 



The Nautilus is the sole " persistent type " among all the varied 

 forms of the Cephalopods, that is to say, it forms the only genus 

 which has been represented in all geological ages since the deposi- 

 tion of the primseval Silurian rocks. The genus underwent some 

 strange vicissitudes. One of the twelve primitive cephalopodous 

 types, it was represented by twelve species in the Lower Silurian 

 seas, these decreased to ten in the succeeding Upper Silurian epoch, 

 and to eight in the overlying Devonian. No less than eighty-four 

 species swarmed in the Carboniferous seas, evidently most favour- 

 able to their development. Of these, all save five were extinguished 

 during the deposition of the Permian rocks, a period of the world's 

 history when life was, comj)aratively speaking, not so abundant in 

 the waters of the earth ; or when perhaps (speaking more correctly) 

 the sedimentary deposits were less favourable to the preservation of 

 the organisms entombed within their now highly-altered strata. 

 Forty-seven species re-appeared in the Triassic and Jurassic forma- 

 tions, while during the Cretaceous epoch the Nautili attained their 

 second maximum point of development, 63 species being recorded 

 from those deposits. These again declined to 15 in the Tertiaries, 

 and to three^ in the existing oceans.^ Ti'uly of most ancient lineage, 

 a contemporary of some of the earliest forms of life, the Nautilus 

 has been a witness of many successive changes in the physical 

 features of the world, and surviving the unknown causes so fatal to 

 all the other members of its race at the close of the Cretaceous 

 period, has held its own in the battle of life, and now forms one of 

 the most curious and interesting of the living wonders of the deep. 



Twenty -five genera of fossil Tetrabranchiates were represented in 

 the Palseozoic seas. Twelve of these were primitive cosmopolitan 

 types, and occur in beds at the base of the Lower Silurian formation 



1 See paper by Dr. S. P. "Woodward on an Ammonite from the Inferior Oolite 

 with the operculum in situ, " Geologist," vol. iii. 1860, p. 328. 



^ As the broad-mouthed Nautilus pompilius is the female, it is highly probable 

 that the N. umbilicatus is the male of the same species. Among Ammonites both 

 tumid and flattened forms have been observed in most species. See " Woodward's 

 Manual of the MoUusca," p. 83. 



3 Cephalopodes, Etudes Gen^rales. J. Barrande. 1877. Table 1, p. S4. 



