6 INTRODUCTION. 
yet unfigured), he was wont to indicate, as illustrative, the near- 
est approximation to it in size and general appearance that he 
could discover. Hence it not unfrequently happens that he has 
quoted figures in 1758 (ed. 10), when his hbrary was less richly 
stored), which eight years afterwards (ed. 12), with a wider ac- 
cess to the costly iconographies (Seba, Regenfuss, &c.) of his 
day, he has either repudiated or has virtually nullified by sub- 
sequent additions. This correction or alteration of his syno- 
nymy renders it very difficult at times to say which of two 
shells, that are alike endowed with the scanty characteristics 
demanded by the text, the one being that represented in the 
majority of engravings cited in the earlier edition, the other 
agreeing with all the new ones quoted in the later publication, 
is to be regarded as the type of the species: in other words, 
whether the laws of priority forbid (an author’s relation to him- 
self being held equivalent to that which he bears to others) that 
he should change or amend that which he has once delivered 
to the public. My own impression is that, although it is not 
expedient to permit the addition of subsequent synonyms to 
affect the name of an object already clearly defined by wholly 
harmonising references, yet that in all cases the annexation of 
further limiting characteristics, provided they be not adverse to 
the earlier ones, should be regarded as explanatory of the ori- 
ginal views of our author, and thus, in respect to descriptive or 
direct definition, the final edition should be regarded as the 
standard: for, although the arrangement of the ‘Museum 
Ulrice’ seems to have been effected so independently of his 
more general work, that the shells designated by the same name 
in these two publications are oftentimes perfectly distinct, it 
does not appear that our author wilfully changed his ideal of a 
species in the editions of his ‘Systema.’ The discrepancies of 
pictorial or indirect definition have resulted from citations of 
illusive engravings, quoted, in default of finding better illustra- 
tions, as the nearest approximations in general aspect to the 
specimens which Linnzeus was describing. 
Wherever the details of a species in the ‘Museum Ulrice’ 
clash with the essentials specified in the ‘Systema’ we must 
recollect that, unless the shell in the Dronningen Museum be 
expressly referred to (as M. U.) in anticipation of its publication 
in that descriptive catalogue, the species defined in the tenth 
