134 MANUAL OF THE MOLLUSCA. 
nay, often seemingly producing results opposed to @ priori 
probability, and at variance with the suitableness of con- 
ditions ?* 
The answer is, that each fauna bears, above all things, the 
impress of the age to which it belongs. Each has undergone a 
series of vicissitudes up to the time when its barriers became 
fixed, and after its isolation it has known no further change, 
but decline. 
The number of living and fossil species of each genus of 
mollusca will be stated in the following pages, so far as they can 
be ascertained. With some modifications, these numbers give the 
following totals, by which the relative numerical development 
of the orders and families will be seen. 
Recent. Fossil. Recent. Fossil. 
CEPHALOPODA. Dibranchiata. Neritidaaticn. .:.00.¢osreee 428 103 
Argonautide ............ 4 2 Patellidze \2.5..2.21s08e. 368 104 
OctOpOdide..cc..cesc0.0ss 63 —_ Dentaliade.............. 50 125 
PREWUNIGE. osscc-etecss eves 104 31 Chitonidae .......2 cece 250 37 
Belemnitide ............ — 140 2 == 
SUETSE7 Ce eae ga aE 30 16 Pulmonifera 8,465 5,819 
ays 2 ulmonifera. 
Betray n PAGOT OW 1D eticias) ee eee 4750 316 
204 189 Limacidze seeeenseneerees 93 4 
Tetrabranchiata. Limnzide deoveccve-csoce Bo2 185 
Hegde ee 6 pea (Marine) sclows ade sesneees a eo 37 
teins am (Ditto, shell-less) ...... 36 _ 
Ammonitidee .........06 — 1600 ‘ 5,404 "542 
mare ee Operculated Pulmonifera. 
6 2193 | Gyclostomidee..........+- 903 45 
GASTEROPODA. Prosobranchiata. Aciculids 2.2, 28 1 
Strombidej ........066. 87 393 — — 
WVEITEICICIS. Fs.05000rs-.000 993 703 931 46 
Buccinidee ...........006+ 1,144 352 Tecti-branchiata. 
OBITS ae eee 856 462 Tornatellida .......+6+. 62 166 
“7EULEG Eanes ieee ae 686 210 SS 168 88 
Cypracida ........0000 227 97 Aplysiad@e ....-sseeees 84 4 
MAA CIGD 2. s52--0-2-25e0e- 268 340 Pleurobranchidee ...... 28 5 
Pyramidellide ......... 216 394 Phy Wdiade veevereereer 14 = 
Cerithiads’.....:3.2:.-.2. 192 610 356 263 
MMelAnIADD .c.20.50s00ce6- 424 50 Nudibranchiata. 
Turritellideet ........0++ 329 CIN Re ee in eee Gs A 160 Al 
Littorinid ..........0606 410 220 Detain ee 38 Fo 
Paludinid2e .........++006 217 110 SRaliine ee 101 ua 
Calyptreide ............ 160 101 Phyllirhoidee ...cse.ee0- 6 — 
Turbinidee ............00 855 906 Elysiade iin iy Claes ak 
HAAN OLIN ceeccsecdveeses 104 136 ——— 
Fissurellidze ........006. 201 76 318 
* Burchell, in Darwin’s Journal, p. 87. ¢ Including Aporriuis 
} With Scalaria. 
