420 PROCEED! NOS OF THE ACADEMY <>K [1893. 



group, and lead to the collection of the numerous forms which it 

 seems reasonable to suppose must attach themselves to the bodies of 

 our many species of crayfish. The monographic study ot such a 

 group as this would doubtless furnish us with data of great value in 

 the consideration of our theories of variation. The writer would be 

 especially indebted to anyone who will kindly furnish him with 

 material collected in other localities or from other species of crayfish. 



Regarding the specific distinctness of the forms here described it 

 may be said that the writer has examined several hundred specimens 

 ranging in size and age from individuals just emerged from the 

 cocoon to those of the largest dimensions, and while each species ex- 

 hibits a considerable range of variation (which will form the subject 

 matter of a subsequent paper), no important transitional forms have 

 been discovered, the diagnostic characters here given being main- 

 tained with as much constancy as is usual amongst closely allied 

 animal species. 



Even should the views of Vejdovsky and Voigt prove to be 

 correct for the European species, which seems probable, this would 

 in no way affect the status of the American forms; for other cases are 

 not wanting where a group of animals which in one region is 

 represented by a single variable species, in another has numerous 

 specific representatives. This would then be only another fact in 

 support of the view, now almost universally conceded, that species are 

 but more widely separated varieties, which under the stress of changing 

 conditions have continued to diverge from a common origin. Indeed, 

 it is not certain that these species should be placed in the same genus 

 even, for they are distinguished from one another by anatomical char- 

 acters of such importance as would, in some groups of animals, rank of 

 generic value. The several species are here referred to the single genus 

 BranchiobdeUa chiefly because the writer is uncertain as to the legiti- 

 mate limits of a genus in zoology, and because in this preliminary paper 

 it is sufficient to recognize the existence of distinct forms without 

 attempting to fix the exact conventional value of the gap which 

 separates them — a labor which belongs more rightly to the system- 

 atise whose work of sorting and arranging would be quite premature 

 in a group of which only a very small portion of the probable 

 number of existing species have been examined. Moreover it should 

 be borne in mind that much confusion has resulted from the un- 

 necessary multiplication of genera. 



