IXTRODUCTION. 
XVI 1 
must, of necessity, be constructed upon characters which would be 
deemed in most cases insufficient for the establishment of the 
latter. 
Until intermediate fossil forms are forthcoming in much greater 
numbers than heretofore, to bridge over the gaps in the palieonto- 
logical series, it is to be feared that very little reduction can be 
made in the number of species that will have to be described. 
In connection with this subject attention may be drawn to the 
remarks made by the late Dr. Xeumayr * on the enormous develop- 
ment of marine life in the Jurassic epoch. We ma}" consider it as 
proved, he says, that the diversity of marine life during the Jura 
epoch was about as great as it is at the present time. Xow the 
Jura Formation contains more than thirty successive zones, each 
with a characteristic fauna; and even if each of these zones has a 
considerable number of species in common with the preceding, and 
with the following zone, the sum of all the forms that lived during 
the whole course of the Jurassic period must have surpassed that of 
the present marine fauna by a large amount, perhaps from ten to 
fifteen times in excess of the latter. It can hardly have comprised 
less than from 500,000 to 750,000 species. Of this overwhelming 
multitude only an insignificant proportion is known. Out of all 
the Jurassic deposits there may be about 10,000 marine animals 
described, a number which must certainly be considered very 
small. 
In the Table of the Xautiloidea contained in Part I. of the pre- 
sent Catalogue, p. xxiii, I inserted the Family Bactritidce. On 
reconsidering the question of the affinities of Bactrites (the sole re- 
presentative of the family), in the light of the investigations of 
Branco^ and Hyatt % I am now prepared to accept the systematic 
position assigned to it by those authors — that is, at the commence- 
ment of the Ammonoidea. Bactrites will therefore be dealt with 
in Part III. instead of in this volume. 
^ Die Stamme cles Tliierreichea, Wirbellose Thiere. Band i. 1889, pp. 21, 
22 . 
2 “ Ueber die Anfangskainnier von Bactrites," Zeitscbr. der Deutsch. geoL 
Gesell. Band xxxvii. Heft i. 1885, p. 1. 
^ “ On Primitive Forms of Cephalopods,” American Naturalist, Jan. 1887, 
p. G4. See also “Genesis of the Arietichc” (Smithsojuan Contributions to 
Knowledge), 1889, Introduction, pp. 1-3. 
PART ir. Ij 
