SONNINIA— SUMMARY. 447 



the strata of what is called the Concavum-zone must have taken an immense 

 amount of time to deposit if the time be reckoned by years ; but it was, of course, 

 short in comparison with geologic time as a whole. Recently, however, I have 

 shown in a paper to the Geological Society (vol. xlix) ! that the strata of the 

 Concavum-zone are divisible into two portions — a lower part characterised by the 

 abundance of forms of the Lioceras-concavum-tjne, an upper part characterised 

 by species of Hyperlioceras. Munier-Chalmas 2 goes further, and appears to speak 

 of three horizons, so that it is very probable that only about one-third of the 

 71 species of Concavum-zone Sonninise were sufficiently contemporaneous to 

 live during what I have for chronological purposes designated "a hemera," 3 and 

 the duration of a hemera I imagine to have been a very considerable space of time 

 from biological considerations. 



Taking, however, the two hemerae during which the strata of what is called 

 the Concavum-zone were deposited, there is reason to believe — not because the 

 species have been obtained in situ, for the greater number of the large specimens 

 were obtained from workmen, but because of matrix and other details — that a very 

 large proportion of the species lived during the discitse hemera ; further, that few 

 of the spinous species lived in this hemera, but that it was characterised by the 

 costate, subcostate, and smooth forms. Further details on this matter will be 

 given in a table of chronological succession which will appear at the end of this 

 volume. 



Table X furnishes to a certain extent an analysis of the Concavum-zone 

 Sonninise according to surface-ornament. It is designed to show readily the 

 forms which, considered merely in regard to such surface-ornament, are morphic 

 equivalents of one another. The principal object of this table is to facilitate 

 identification ; as a classification it is, of course, of no value at all, and must not 

 be confounded therewith. 



The Table is divided into six main columns according to the ornamentation 

 which these Sonninise possess, namely : — I. Tnberculate, which speaks for itself. 

 II. Tuberculate-costate, which includes those species wherein tuberculation is 

 retained until a fairly late stage of ontogeny, so as to form a marked feature of the 



horizontally they are seen to be merely slowly deposited representatives of thick beds elsewhere. 

 Further, they are found to contain several distinct successive faunae indicative of much biological 

 evolution, itself demanding a very considerable length of time. For instance, the Cotteswold 

 Cephalopod-bed, which was no doubt in the minds of Morris and Lycett, contains five distinct 

 Ammonite-faunae at the least, whereby it is shown to be contemporaneous with several hundred feet 

 of strata in other parts of this country and on the Continent. 



1 "The Bajocian of the Sherborne district." 



2 " L'Etude preliminaire des Terrains jurassiques de Normandie," ' Compte rendu sommaire des 

 Seances, Soc. geol. France,' No. 14, 1892. 



3 ' Quart. Journ. Geol. Soc.,' vol. xlix, op. cit. 



