arte / 
162 MONOGRAPH OF BRITISH LAND AND FRESHWATER MOLLUSCA. 
FAMILY ARIONIDEX Gray. 
The family Arionide, according to Pilsbry, 
is somewhat discontinuous in its geographical 
range, occupying three widely separated areas, 
in each of which a predominant type occurs. 
The West American area has the greatest 
number and variety of genera, embracing the 
Binneyinw, Ariolimacine, etc., and forming 
the most primitive group of the family; one 
form, Binneya, possessing a spiral external 
shell with sculptured nepionic whorl, short 
body cavity, and solid tail, may be regarded 
as linking Arionidw with the Endodontida, 
from which they are supposed to have been 
derived. 
The Asiatic centre is concentrated in the 
Himalayas, and represented by Anadenus, a 
group in which the caudal gland is wanting 
and the male intromittent organ still present, 
undoubtedly representing in these respects the 
most ancient form of the group. It is most 
Fallo A er a_ Closely allied to the genus Prophysaon, but in 
the penial development shows nearest affinity 
Hesperarion, both of which are now West American groups. 
The true Arions, the most highly developed forms, have their home in 
the European region, the probable source of origin of the entire group, from 
whence in past. ages the earlier and more primitive genera have spread 
throughout the northern hemisphere, the most simply-organized groups, as 
is usual, occupying the regions most remote from their place of origin, and 
not, as is too prevalently ‘believ ed, persisting in their evolutionary centre. 
The Avions are remarkable for the peculiar penial degeneration they have 
undergone, and the assumption of the intromittent function of that organ 
by the oviducal passage. 
The Arionidw are not descended directly from the primitively shell-less 
forms, as has been averred, but unmistakeably show their descent from a 
group with well-developed spiral shells, the American forms supplying the 
chief links which make plain the progress of the modifications and clearly 
demonstrate that the typical genus Arion is the terminal member of a 
series of forms beginning with Binne ya, half-slug and half-snail with 
almost helicoid muse -ulature, and passing by numerous intermediate stages 
still existing to the typical Arion organization. 
The family is probably most satisfactorily divided by utilizing the various 
modifications of the free-retractor muscles, their arrangement showing also 
the weightiest differences between the Arionidw and other slugs. 
With this group Signor Carlo Pollonera, of Turin, is here associated in 
cordial recognition of the extent and importance of his researches upon the 
organization, specific differentiation, and classification of the Arionidw 
and of the slugs generally. 
In the British Isles the family is represented by only two genera, Arion 
and Geomalacus, the remaining and more ancient groups beimg now 
restricted to the remoter parts of the northern hemisphere. 
