238 PRINCIPLES OF HYDROID ZOOGRAPHY. 



must keep in mind, however, that the name, when used in this sense, is purely provisional and 

 liable to be clianged when the discovery of the trophosome shall determine the true geims of 

 our, then no longer incomplete hydroid. So also no name which has been given to a tropho- 

 some whose gonosome is unknown can be regarded as otherwise than provisional. 



It is upon these principles that I have based the diagnoses, nomenclature and arrangement, 

 of the families, genera and species, of hydroids, to whose description the second part of the 

 Monograph is devoted. 



Keeping in mind that it ought to be the aim of the systematist, in framing his diagnosis, to 

 render it al^solutely exclusive of every object or group of ol)jects to which it is not intended to 

 apply, I have endeavoured in every case to carry out this principle. With the view also of secur- 

 ing against difFuseness, it has been my aim in the definitions of the more special groups to avoid 

 a repetition of those characters on which the more general groups are based. In other words, I 

 have endeavoured to confine each diagnosis to its proper " differentia." It is true that the con- 

 ception of any group involves, not only the " differentia '' of the group, but also the characters of 

 those more general groups imder which it is included. These characters, however, instead of 

 being repeated at length, may in our technical diagnosis be expressed by the use of a single word — 

 that which has been adopted as the name of such higher group as immediately includes the 

 group under definition. This word would, then, like an algebraical expression, become a short 

 symbolic representation of a multitude of distinct facts, the facts which constitute the essential 

 characters of the group of which it is the accepted name, and which is supposed to have been 

 previously defined ; and though it may not be deemed necessary actually to express this word in 

 the definition, it or rather the facts it represents must in every case be understood. 



In order to facilitate comparison I have adopted as far as possible a uniformity in the selec- 

 tion of characters, and in the order in which these characters are noted, while in every diagnosis 

 I have given one paragraph to the trophosome and another to the gonosome. 



When the colour of the hydroids composing a species is known I have always given it. Colour 

 affords a character which cannot be neglected in our specific descriptions, but as it is in many 

 cases too variable to be relied on with certainty I have not ventm-ed to introduce it into the 

 proper diagnosis. 



The technical terms employed in the descriptions are not numerous, and, if these be not 

 immediately understood, there will be no difficulty in rendering them so by a reference to the 

 section on Hydroid Glossology (see part I, pages xiii, &c.), where every term is defined. By the 

 use of such terms I have hoped to avoid tedious circumlocution, and to condense the descriptions 

 without sacrificing their precision ; and if in some cases the definitions may appear more diffuse 

 than could be desired, they do so because I believed that condensation in such cases could not 

 have been carried further consistently with exactness. The beautiful simplicity and terseness of 

 the Linnsean definitions is scarcely possible any longer ; for the vast increase of known species 

 since the days of the great systematist renders necessary the employment of a very much larger 

 number of characters for exact diagnosis than sufficed at a time when comparatively few species 

 had to be distinguished from one another. 



The follow ing table will show the mode in which I have distributed the genera of gynnioblastic 

 hydroids under their respective families : — 



