MONOGRAPH OF THE LEUCOSIAD^. 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 particvxlarly the case in respect to the present family, of which Leach 

 gives many new genera and species in his valualjle repertory the ' Zoological Miscellany;' 

 but in so slight a manner that only tlie tickets applied to the specimens in the Museum 

 by Leach liimself 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 objectionaljle. 

 To describe an organ in one species as longer or broader or tliicker 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 distiactive 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 

 som'ce of errors in synonymy, and the cause of interminal)le and unsatisfactory research 

 and labom*. 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 description 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 woi-k 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 of 

 their organization as renders it unnecessary for me to enter into any considerable detail 

 of their structure, particularly as that work is La the hands of every one interested in this 

 subject. But in the fine work of De Haan on the Crustacea of Japan, there occurs so 

 concise and clear a summary of the characters of those organs on which the classification 

 depends, that I will venture to quote the passage at length : — 



" Regiones pterygostomianse supra palpos maxillarum quuitarum excavatae, extrorsum 

 marginatse, excavatione parallela usque ad oris apicem producta ; maxiUarum quintarum 

 articuli secundi et tertii ter longiores quam lati[ores], triangulum deseribentes, margine 

 interno sunt obtusi ; maxillarmn quintarum lacinise externse maxLUis quintis sequales et 

 palpomm flagella tenuissima; maxillse secunda? minimjB, laciniis externis cum palpis 

 coaUtis, et setis duabus a sibi invicem apice distinctis ; sella turcica brevissima vel nulla ; 

 apodemata sterno intermedio distantia ; branchiae sex, nulla maxUlis, unica pedibus tertiis ; 



