22 
Transactions of the Society. 
for the retention of Linnaean genera, and in his ‘ Zoological Classifi- 
cation ’ he gives Acarus as equivalent to Tyroglyphus, thus taking 
the common Cheese-Mite as the type. Probably it has most frequently 
been considered so, because it is very well known ; unfortunately it 
has an equally well-known rival in the Itch-Mite, which many other 
writers took as the type, besides other competitors ; and thus hopeless 
confusion arose. The fact is that the simple circumstance, that the 
knowledge of this particular group of creatures had made so little 
progress in Linnaeus’ day that he was forced to include all in one 
genus, is not a sufficient reason for endeavouring to keep up a type 
genus of Acarina when we do not attempt it in the case of Insects, 
Crustaceans, or other similar divisions ; and accordingly no trace of 
the genus Acarus will be found in the classifications of modern 
specialists ; the genus has expanded into an order or class, and its 
identity as a genus has been merged, just as a supposed species 
which is found to include several forms is lost when its specific name 
is expanded into a generic one. If, however, the genus be lost it is 
naturally different with the Linnaean species, they are carefully pre- 
served ; but it is well to understand what a Linnsean species means in 
such creatures as the Acari. The botanist, if in doubt as to what was 
the exact plant to which the Linnaean name was given, can usually 
refer to that naturalist’s own Hortus siccus in the possession of the 
Linnean Society, and examine the plant itself, and, unless the speci- 
men has been injudiciously interpolated at a later period, Linnaeus’ 
well-known handwriting will be found on the mounting-paper. The 
students of many of the departments of zoology are more or less in 
the same position, but no types have been preserved of such minute 
creatures as Acari ; indeed so entirely has their small size hindered 
the retention of types, until quite lately, that although Nicolet’s 
‘ Oribatidae of the neighbourhood of Paris,’ published in 1855, was 
written for and appeared in the ‘ Archives du Museum d’Histoire 
Naturelle ’ of that city, not a single type of any of his species was pre- 
served either in that museum or elsewhere, and when I wrote my 
book on the Oribatidae, I was not able to obtain a sight of one of 
Nicolet’s specimens. Thus for the Linnaean species we have to rely 
upon his descriptions, but when we refer to them it is quite impos- 
sible to identify Acari by them. I will take an example. There is 
an Acarid probably known to most of you called Gamasus crassipes , 
it is a Linnaean species ; but the Linnaean description would apply 
equally well to any one of dozens of species of Gamasidae, or to any 
one of hundreds of other species belonging to totally different families. 
Schrank’s description in 1781 might possibly identify the family, but 
Hermann’s in 1804 is the first which would give any chance of iden- 
tifying the species. Thus a Linnaean species of Acarus really means 
what the first writer who gave a description or figure from which 
identification is possible supposed to be the Linnaean species. 
Now let us look at what, under the influence of modern investiga- 
