HILDOCERATIDtE. 223 



my original limits in order to give a more complete account of the members of 

 those genera. I shall have to repeat this offence on other occasions. 



So far as the Rildoceratidse are concerned I have completed my task — at any 

 rate for the present. That there are many matters in which the knowledge 

 gained during the progress of this work indicates that improvements might be 

 made T very readily confess. There are many points connected with the 

 genealogy of the species of the Hildoceratidse which are very obscure ; and it is 

 only too probable that the advent of fresh specimens and new species may 

 necessitate certain alterations in the general grouping of the family, by furnishing 

 clearer evidence of the genesis of the different species. No doubt in the future I 

 shall have to add a supplement upon the Hildoceratidse, and then I shall hope to 

 treat of the whole family, and not merely, as in this work, of a portion separated 

 by a more or less artificial geological division.^ 



The family Sildoceratid.se was dominant from the Falciferum- to the Concavum- 

 zone inclusive. The species which lived prior to the time of the Falciferum-zone 

 were few in number, and, in a geographical sense, belong more especially to the 

 area in contact with the Mediterranean. It would seem as if this family, as well as 

 others, made that district their head-quarters, and that from thence successive 

 waves of less-modified species were constantly being sent forth. As these species 

 spread they ousted their more changed predecessors ; while, after having in their 

 turn become modified, they were themselves ousted by the species of the next 

 wave of Ammonite-immigration — members, possibly, of the same family. 



In this manner the genus Harpoceras, which dominated the Falciferum- 

 and the Gomnmne-zone, was ousted by Grammoceras, which dominated the Jurense- 

 and the Opalinum-zo-ae ; and the latter in its turn was ousted by Ludioigia and 

 Lioceras, which dominated the Murchisonse- and the Goncavnin-zone. 



The ideal to which the Hildoceratidas tended, was the compressed, involute 

 shape. Thus, for instance, if we take only the species described in this Mono- 

 graph which obtained this shape, we have Ludioigia cornu — the genera Lioceras, 

 Pseudolioceras, and Eyperlioceras, — Haugia Eseri — Gravvmoceras subserrodens, 

 leurum, and CoUesivoldise — and Polyplectus discoides. It may be suggested that this 

 shape was, in reality, a necessity. When the evolute form became dominant it 

 caused over-crowding, brought about a scarcity of food, and induced greater 

 economy, which resulted in the compressed, involute shape. The involute forms 

 therefore possessed a distinct advantage over the evolute forms of their own genus, 

 and consequently they survived while the latter died out. When, however, from any 

 cause they were brought into conflict with a fresh immigration of evolute — and 

 therefore presumably vigorous — forms of another genus, which had not felt the 



' See my paper " On the Cotteswold, Midford, and Yeovil Sands," etc., Quart. Journ. Geol. 

 Soc.,' vol. xlv, pp. 461— 4G8, &c. 



