196 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



the present. With the exception of two species now living, they are extinct, and the pos- 

 sibilities of their structural combinations were exhausted even before the extinction of the 

 Cretaceous fauna. The shells, also, are not simple, like those of others of their own sub- 

 kingdom, but are provided with a series of partitions or septa, which afford additional 

 means of verifying the observations made upon their superficial characteristics. The ob- 

 jections usually urged against the accuracy of investigations founded solely upon the shell, 

 cannot, therefore, have the same weight, when they are supported, as in the Tetrabranchi- 

 ates, by the evidence of internal structures. In some respects shells are the most satis- 

 factory of all animal remains. While the bones of the extinct Vertebrata, the casings of 

 the Echini and Crustacea, and other fossils, give us in each specimen but one period or 

 stage of growth, shells embody in nearly every well-preserved example a history of the 

 individual from its earliest period to its death. The shell, indeed, is but a single organ, but 

 then it retains the impression of the contour and many of the minor peculiarities of the 

 softer parts, and when we have another set of internal hard partitions of acknowledged 

 importance, there is but slight danger of overrating the value of their evidence. 



The Nautiloids commence in the Silurian with aberrant genera, such as Orthoceras, 

 Phragmoceras, Gyroceras, Lituites and others, and these gradually die out until they are 

 no longer to be found in the Jurassic. The normal discoidal Nautili begin in the Upper 

 Silurian with two species ' and gradually gain in numbers, amounting in the Carboniferous 

 to upwards of thirty-eight species, and then decrease until but two species survive in the 

 seas of the present day. The greatest variety in the form of the adult whorls and the 

 most complicated ornamentation is exhibited at this epoch, when the number of the species 

 and the vital energies of the series are at their maximum. There are globular, keeled, 

 channelled, flattened, smooth and tuberculated species, — all those essential differences of 

 the adult whorl which vary its form but one, the complete involution of the spiral. Subse- 

 quently these ornamented and keeled species gradually disappear before the striated and 

 ribbed Nautili of the Jura and Cretaceous, and the latter before the smooth Nautili, which 

 have been slowly gaining in number and importance ever since their first appearance in 

 the Silurian. There are only six of these in the Tertiary, whereas there are about eleven 

 smooth species, according to Giebel, in the Cretaceous. 2 Thus while their predominance in 

 the Tertiary is significant of the extinction of the more highly ornamented species of the 

 Nautili, their falling off in number shows the whole of the Nautiloids to be approaching 

 their dissolution. 



The same laws, however, do not obtain with regard to the greater or less involution of 

 the shell and the complication of the septa. The Nautili of the Carboniferous and pre- 

 ceding geological formations are not involute, but those of the Jura and succeeding forma- 

 tions have the umbilicus more or less covered by the enveloping whorls ; the septa are 

 reciprocally progressive, and attain their highest complication in the angular lobes of 

 Nautilus ziczac of the Tertiary. Thus during the decline of the Nautiloids, both in the 

 number and in the variety of the form and ornamentation of their whorls, the very 

 species that indicate this decline are becoming more complicated in respect to the in- 

 volution of the whorls and the formation of the lateral lobes. 



This sustained progression corresponds precisely with what may be observed among the 

 individuals of the series. From the earliest period they perpetually lengthen the radii of 



1 Mentioned by M. Barramle in " Etage E " of Bohemia. Quart. Journ. Geol. Sue, London, Vol. X. (Translation). 



2 Giebel, Fauna der Vorwelt, Ceph. iii. 1852. 



