Xx INTRODUCTION. 
The Glandinw were formerly widely spread over the European region, 
and also lived in this country, and though still meagrely represented in 
Southern Europe, are now practically expelled to Central America and the 
West Indies, where the metropolis of the genus is now located, and 
where living species have been found scarcely distinguishable from the 
fossilized shells of our Oligocene strata. 
It has been suggested that Limav and Milux have their immediate 
derivation from /Tyalinia, but it is by no means certain that this is the 
case with MWilar, Prof. Babor remarking on their strong afhmity with 
Heliv. he more distant ancestors cannot be indicated with any approach 
to precision, but it is certain that they would possess a more substantial 
shell than the Hyaliniw now possess. ‘The true Limacidw retain the soft 
and supple body, and the degeneraie shell, though reduced to a flat and 
almost un-nucleated calcareous plate buried beneath the mantle, is still 
present. The group is almost restricted to the Palearctic zone, the most 
advanced and recently evolved species occupying a compact and compara- 
tively restricted area, while the more ancient and primitive species have 
achieved a wider dispersal, but have been more or less completely expelled 
from the most active evolutionary region, or compelled to resort to and 
become isolated in undesirable spots. 
The Avrionidw are believed to have descended from Endodontoid 
ancestors, a group which, though world-wide in distribution, has now 
withdrawn its headquarters or metropolis to the islands of the Pacific 
Ocean. 
The typical genus Avvon is naturally restricted to the Palearctic region, 
extending from Portugal to Siberia. It displays the practical completion 
of the process of shell degeneration, and will in all probability be followed 
by a gradual calcification of the outer integument, as a dense deposition of 
lime particles and spicula within its substance has been already initiated, 
and in Avion ater las imparted a certain stiffness and rigidity to the 
body and to its movements; this being the first stage in the slow evolution 
of another complete shelly protection. ‘That this, or a similar process, has 
probably occurred in the past, is demonstrated by the vestiges still present 
in certain species of mollusks of previously existent primary or primary 
and secondary shells or “Protoconchs,” which are the last remaining evi- 
dences of more perfect shells once possessed and successively lost, but 
which had undergone, untold ages ago, cycles of development and degenera- 
tion similar to that many shells are undergoing at the present day. 
The genus Geomalacus is apparently now confined to Western Europe, 
and to the south-west corner of Ireland, which is a last foothold for many 
plants and other forms of life on the verge of extinction in this country. 
