DISTRIBUTION OF MOLLUSCA IN TIME. 127 
DEVELOPMENT OF FAMILIES, GENERA, AND 
SPECIES, IN TIME. 
ao |e | a : 
Sega | Mesa ead ales Total 
GEOLOGICAL ig Bie] & © || Number of | 3 
SYSTEMS. Genera.| “¢ | 3 || 4 || Species fis 
| 2] s | & ||@?Orbigny). I 
Oo};/O)]RA]0 Fy 
2 CALMTLAG, .svocncconss ness 49 | 12) 11) 15) 11 362 18 
S PSultMIa ING piece sieeve ceneces 53 13 | 11 | 16 | 13 317 20 Al 
See DEY ODIAN.......cperace2ee 77 «(| 14 | 20 | 23 | 20 10385 24 32! 
3 § Carboniferous............ 79 | iL | 26 | 19 | 23 835 z | ' 
a PA CUMIAM Es ee ataiseiomesialeo so 66 6 | 24 | 16 | 20 74 30 
ET TUEIAB occ scecueciedseees gi | 9| 25/16] 31 m3 | 35) 
= 5 MS SURASSIC: .cccciscesnavces 107 12 | 35 | 12 | 48 1502 42 
B De IMCASSIC eh so. georeans 108 13 | 36 | 9] 50 1266 49 b 57 
Sates L. Cretaceous............ 123 20 | 41 9 | 53 784. 52 
Q Ui Cretaceous.......0...: 148 16 | 59 | 14 | 59 2147 56 
nM : eee ag Ve = ||| ee ees 
re PIDCENIG cs cessesacenessesees 172 Aa ase le tile 72s 2636 60 
< 7 Miocene Maleate deme 178 2 ll ay i) an |) 703 9249 60 C_ 
te POCEDE! 5 <.scks eecaesese 192 1 |160 | 12 | 79 437 62 (18 
= TREES QSesonbpoeasenanpnoda 400 21 /251 | 13 |115 16,000 78 
SI Eas eet ees Si 
| Recent & Fossil......... 520 56 |280 | 34 |150 30,000 85 | 
Order of Appearance of the Groups of Shells.—The first and most 
important point shown in the preceding Tables, is the co- 
existence of the four principal classes of testacea from the earliest 
period. The highest and the lowest groups were most abundant 
in the paleozoic age; the ordinary bivalves and univalves 
attain their climax in existing seas. If there be any meaning 
in this order of appearance it is connected with the general 
scheme of creation, and cannot be inquired into separately ; but 
it may be obseryed that the last-developed groups are also the 
most typical, or characteristic oy their class (p. 49). 
The Cephalopoda exhibit amongst themselyes unmistakable 
eyidence of order in their appearance and succession. The 
tetrabranchiate group comes earliest, and culminates about the 
period of the first appearance of the more highly-organised cuttle- 
fishes.t The families of each division which are least unlike 
* Those genera are estimated as belonging to each system which occur in the strata 
both above and below, as well as those actually found in it. We have left this table as 
it stood in the first edition, as we are unable to correctallthe figures. This, however 
is not of much importance, since the main points, such as the gradual increase in the 
number of families, would not be affected. 
+ The Palzoteuthis of Broun (not D’Orb.) appears to be a fish-bone, from the equ:va 
eut of the Old Red sandstows in the Eifel. 
