INVERTEBRATE PALAEONTOLOGY. 495 



that, as .a whole, the Silurian species fall more properly into the typical sec- 

 tion, including the existing species, than into any of the other subdivisions 

 that were developed in the Carboniferous, Triassic, and some later periods. 



In the Carboniferous rocks, we meet, with the Temnochilus, Tremato- 

 discus, Discites, Solenochilus, and other aberrant sections, along with species 

 falling into the typical division. In the true Permian rocks, the genus 

 Nautilus seems to be little developed, and the known species most nearly 

 conform in general to the typical section.* 



The Triassic rocks contain quite a diversity of types of this genus. 

 Here we find, with the typical section, Discites still represented ; but Temno- 

 chilus, Trematodiscus, and Solenochilus seem to have died out before the 

 commencement of this epoch. Here we also apparently meet with the Her- 

 coglossa section for the first time, or at least with nearly allied types; and 

 also several aberrant forms that apparently do not fall properly into any of the 

 foregoing sections. 



Again, in the Jurassic rocks, the typical section of Nautilus is well devel- 

 oped, along with forms approaching Discites, and others with strongly- waved 

 septa, more or less nearly like Hercoglossa. In the Cretaceous, we also have 

 typical Nautilus still more extensively developed, along with Hercoglossa, 

 Pscudouautilus,\ and a very few forms like Discites; while nearly all of the 

 other sections seem to have died out. In the Lower Tertiary, we still meet 

 with Hercoglossa, but it also soon disappears, to be replaced by the genus 

 Aturia, which mainly differs in its funnel-shaped, larger, and more nearly 

 internal siphuncle. Both of these types, however, died out near the close 

 of the Eocene, leaving the typical section of the genus as its sole repre- 

 sentative through the more recent formations, and in our existing seas; in 

 which latter it is represented by so few known species as to lead to the con- 

 clusion (in view of its former great development) that the genus may probably 

 be regarded as approaching its end. 



* This remark is, of course, not intended to apply to a very peculiar type described by Professor 

 Geinitz, from tbe Permian rocks of Germany, under the name Nautilus Seebachiamts. This shell has the 

 very small, globose, involute portion without an open umbilicus, and the very large body, or outer cham- 

 ber, straight, with its inner side open, and its lateral margins spread out like great wings on each side. 

 This I have elsewhere proposed to call PteronauUlus. It forms a very marked and distinct genus. 



t This type, I believe, orly occurs in the Neocomian, the oldest member of the Cretaceous. 



