a BRITISH MOLLUSKS. 



proper by the presence of a pore or gland, for the more copious 

 secretion of mucus, at the extremity of the tail, and in having the 

 pulmonary sac and overlapping shield nearer the head, with the 

 respiratory orifice in front ; and the shield has no internally deve- 

 loped shell, its place being occupied by merely a few calcareous 

 grains, which are sometimes isolated, sometimes aggregated into 

 a rude irregular mass. But there are good generic characters in 

 Avion, as distinguished from Limax, apart from the position of the 

 pulmonary sac and the want of a symmetrically-formed shell. The 

 body is enveloped by integuments of more considerable density, 

 rising into wrinkie-hke tuberosities or leaflets, and there is no dorsal 

 keel. Important anatomical differences are also recorded. 



We have only two undoubted species of Avion in this country 

 A. atev and hovtensis. A third, A. Jlavus, has been described, and 

 I have living specimens before me, obligingly communicated by 

 Mr. E. J. Lowe, from the vicinity of Nottingham, as well as from 

 Mr. Bridgman of Norwich ; but M. Moquin-Tandon includes the 

 species in his list of ' Especes Incertaines,' and I cannot bring my 

 mind to the conviction that it is anything more than a yellow dwarfed 

 variety of A. Jwvtensis. Notwithstanding the record given by Mr. 

 Jeffreys in his ' Gleanings,' that Mr. Bridgman finds A. Jlavus on 

 horse-chestnut leaves near Norwich, I am favoured with that gentle- 

 man's opinion, that it has always been a question with him whether 

 A. Jlavus may not be the young of A. hortensis. Species are much 

 too readily named. As many as ten species of Avion have been de- 

 scribed in France alone, four of them within the last few years by 

 M. Millet and M. Normand ; but the only species since recognized 

 by Moquin-Tandon in addition to our A. atev and hovtensis are A. 

 albus and subfuscus, which are apparently varieties of the first- 

 named. Three new species have also been described by M. Mo- 

 relet, from Portugal. The only Avion observed in the New "World 

 as indigenous is a species collected near Discovery Harbour, Puget 

 Soiind, A. foliolatus, Gould. A. hortensis has been transported 

 to the United States, and has become acclimatized within a limited 

 range in the vicinity of Boston. 



The British species of Avion are : — 



1. ater. Animal stout, three to five inches in length, mostly of 

 a dark chocolate colour inclining to black, integuments rising 

 into oblong tuberosities. Shell represented by calcareous 

 granules, sometimes rudely agglomerated. 



