CHAP. XXII.] 
MOLLUSCA. 
537 
Family 20.— GASTROCJDENID.E. (5 Genera, 40 Species.) 
Distribution. — Temperate and warm seas. Aspergillum ranges 
from the Eed Sea to New Zealand. There are 35 fossil species, 
ranging back to the Lower Oolite. 
Family 21. — PHOLADIDiE. (4 Genera, 81 Species.) 
Distribution. — These burrowing molluscs inhabit all Temp e- 
rate and warm seas from Norway to New Zealand. There are 
about 50 fossil species, ranging back to the epoch of the Lias. 
General Remarks on the Distribution of the Marine Mollusca. 
The marine Mollusca are remarkable for tlieir usually wide 
distribution. About 48 of the families are cosmopolitan, rang- 
ing over both hemispheres, and in cold as well as warm seas. 
About 15 are restricted to the warmer seas of the globe ; but 
several of these extend from Norway to New Zealand, a distri- 
bution which may be called universal, and only 2 or 3 are 
absolutely confined to Tropical seas. Two small families only, 
are confined to the Pacific and Indian Oceans. Marine fishes, 
on the other hand, have a much less cosmopolitan character, no 
less than 30 families having a limited distribution, while 50 
are universal. Some of these 30 families are confined to the 
Northern seas, some to the Atlantic and Mediterranean, and a 
considerable number to the Indian Ocean and Western Pacific. 
Many of these families, it is true, are much smaller than those 
of the Mollusca, which seem to possess very few of those small 
isolated families of two or three species only, which abound in 
all the Vertebrate classes. These differences are no doubt con- 
nected with the higher organisation of fishes, which renders them 
more susceptible to changed conditions of life ; and this is indi- 
cated by the much less antiquity of existing families of fishes, 
the greater part of which do not date back beyond the Cretaceous 
epoch, and many of them only to the Eocene. In striking con * 
trast we have the vast antiquity of most of the families of Mol- 
