MONOGRAPH OF THE LEUCOSIAD.E. 279 
a species should be as tangible and certain as possible, but that every new species should 
be fully, as well as accurately described in detail. In this respect Leach was generally 
very deficient. The only indications left of many of his species and even genera, consist of 
half-a-dozen words, in many instances so vague that they have become useless, and, as 
they are often not illustrated by figures, it would be impossible to identify the species to 
which they refer, were it not for the existence of the specimens themselves in the British 
Museum. This is particularly the case in respect to the present family, of which Leach 
gives many new genera and species in his valuable repertory the ‘ Zoological Miscellany ;’ 
.. butin so slight a manner that only the tickets applied to the specimens in the Museum 
by Leach himself afford any sure index to the species intended. 
.. This however is not the only fault to which naturalists are prone with regard to the 
definition or description of new species. Nothing is more common than that the only 
characters given are deduced from a comparison of the development of certain organs with 
the same parts in nearly allied species. Such specific characters are always objectionable. 
To describe an organ in one species as longer or broader or thicker than the correspond- 
ing organ in another species for instance, infers the necessity of an actual comparison of 
one with the other, which, of course, is often impossible. Specific distinctive characters 
should always be either absolute, or derived from points of comparison within the indi- 
vidual itself. Another defect from which much confusion has arisen, is the want of a full 
detailed description of each species, which is necessary, however nearly it may be allied 
to another; and this should include every organ of importance that can be easily and 
certainly brought under review. The want of this desideratum has been the fruitful 
source of errors in synonymy, and the cause of interminable and unsatisfactory research 
and labour. I may be allowed to add, that the specific definitive phrase should be such 
as to point out, as briefly as may consist with clearness and certainty, the points of 
_ distinction from all those already known in the same genus; whilst the gosonriphon should 
be so full as to enable the naturalist to ascertain whether any individual afterwards 
observed is a.new species, or identical with that described. 
The admirable work of Professor Milne-Edwards, which has been the text-book of every 
student of this class of animals ever since its publication, contains such a general view 
their organization as renders it unnecessary for me to enter into any ep 
of their structure, particularly as that work is in the hands of every one gem - 
subject. But in the fine work of De Haan on the Crustacea of J apan, : isis ed = 
concise and clear a summary of the characters of those organs on which the c 
depends, that I will venture to quote the passage at length d viol cilia 
. * Regiones pterygostomianæ supra palpos maxillarum T TREE 4 quintarum 
marginatæ, excavatione paralielà usque ad oris apicem poems Tv EUN MNT 
articuli secundi et tertii ter longiores quam lati[ores], triangulum «esor Dan nin à 
interno sunt obtusi; maxillarum quintarum laciniz Á— ds piana cum palpis 
palporum flagella tenuissima; maxille secundæ minimæ, laciniis 
istincti: i issima vel nulla; 
coalitis, et setis duabus a sibi invicem apice distinctis ; sella vet BEL saitas "em 
apodemata sterno intermedio distantia ; branchiæ sex, nulla ma a 
