1887.] NATURAL SCIENCES OF PHILADELrPIIA. 171 



ing forms that no description could distinguish, and no one of which 

 would probably entirely correspond with the next specimen collected 

 by its author from the same stream. This variability in forms that 

 were considered typical when the first of a species was collected and 

 named, is a fact in science that will not fail to impress any who 

 may gather from many waters and through a wide extent of country. 



Among some very incomplete memoranda of my collections and 

 receipts, I find it recorded that I have examined Spongilla fragUis 

 from at least 32 localities in 18 North American Stat(5s; S. lacusti'is, 

 from 26 localities in 16 states; Meijenia flaviatilis from 25 localities 

 in 14 states; Tubella pennsylvanica from 18 localities in 11 states etc. 

 Had a perfect list been kept, the figures might be largely increased. 

 And this is the lesson most obviously taught : hardly any two 

 specimens are exactly alike in their so-called typical features ; but 

 all may be grouped, as in the case of those brought together under 

 any of the above designations, and common definitions or descrip- 

 tions will, without undue elasticity, cover them all. The varieties 

 I have retained are such as were originally considered good species, 

 and have generally some slight j)eculiarity to recommend them ; but 

 to the student I would say, " Get your genus right and your species 

 right, and then it will matter little whether you associate it with 

 any variety." 



The following "Diagnosis" was prepared at my request by Prof 

 Franz Vejdovsky, of the University in Prague, Bohemia, to give 

 information as to the number of reliable species of fresh-water 

 sponges known by students at the present time to inhnbit Euro- 

 pean waters, with their proper synonomy etc., and is far more 

 reliable than I could hope to make it from the scattered literature 

 of the subject. Professor Vejdovsky has greatly aided my work by 

 thus furnishing, in manuscript, a German translation from his Bohe- 

 mian text; for the English version of which I am further indebted 

 to my friend Prof. Benjamin Sharp of the Academy of Natural 

 Sciences of Philadelphia. As the classification adopted differs from 

 that of Mr. Carter, which, for many reasons, I prefer to follow, it has 

 seemed to me best to present the paper as a whole in this place, in- 

 stead of collating the species and distributing them amongst my 

 descriptions. Due reference will, however, be made to all in their 

 proper order according to Carter's system. 



