62 UNIVALVES. 



shape; those of Accra and Bulla are not unlike that of Limncza in 

 colour and material ; those of Cylichna and Tornatinus are small and 

 distinctive ; those of Philine, Aplysia, and Pleurobranchus are mere 

 membranes. The sea- slugs have no shells, and, as in their case it is 

 not worth going over the same ground twice, we will leave their 

 points of identification to be dealt with under their families and 

 genera further on. 



The land-slugs have shells, but they are internal and rudimentary, 

 and dwindle down to a few granules, to show that a shell was 

 possessed by distant ancestors. The easiest way to identify the 

 slugs is by their breathing aperture. In Geomalacus, the spotted 

 slug hailing from the south-west of Ireland, which is the connecting 

 link between Limax and Arion, the aperture is near the front of the 

 shield. In Arion it is in the middle of the shield. In Limax and 

 Amalia, a genus formerly included with Limax, it is near the back of 

 the shield, Amalia having the mantle granulated, Limax having it 

 striated. If you are in doubt as to which is the mantle, you have 

 only to irritate a Limax to see him duck his head under his mantle, 

 or catch him asleep, when you will find he has put his mantle over 

 his head to make himself comfortable. 



It will be noticed that we have grcTuped our land slugs under five 

 genera Testacella, Amalia, Limax, Arion, and Geomalacus ; and of 

 these genera recorded seventeen species. Had we included varieties 

 we should have had fifty more figures, and even then we should not 

 have illustrated them all or made clear the difference between them. 

 Had we given the varieties of all the species, marine and non- 

 marine, we should have required two thousand illustrations, to 

 include only those distinctive enough to be worth regarding. 



Among the molluscs it is more difficult than usual to draw the 

 line between a variety and a species, or even between one variety 

 and another. Several of the species are old varieties promoted to 

 the higher rank, and, as a rule, it is desirable to keep new forms 

 among the rank and file until the reasons for their promotion are 

 strong enough to satisfy the reasonable. As instances of promotion, 

 we have Helix hortensis, looked upon for years as a variety of Helix 

 nemoralis, and as an example of the intermediate stage, we have 

 Testacella scutuhtm, accepted as a species by some, and retained by 

 others in its old place as a variety of Testacella haliotidea. 



Varieties are the general collector's horror and the specialist's 

 delight. To have dealt with them in this book would have made it 

 too heavy in two senses, and so we restricted it to the species only, 

 and we have also simplified the path of identification by keeping 

 clear of sub-genera, which are rather as luxuries than necessities. 

 Dealing with one country's fauna, as we do, the old genera are not 

 too large for our purpose ; but dealing with the world's fauna, such 

 a genus as Helix, for instance, becomes too crowded with species to 

 be wieldy, and the species sort themselves into unmistakable groups 

 for which a collective name is manifestly a convenience. Such 

 names have, consequently, been adopted, and, in due course, have 

 found their way into our local lists. They would be used in all 

 cases if there were any indications of permanence amongst them, but 

 of the making of sub-genera, as of the making of species, there seems 

 to be no end. Hence the generic name is usually retained, and the 



