118 University of California Publications. [Zoology 



The size of the organism is also of aid in distinwnishinsr the 

 species. This is shown very strikingly on a comparison of draw- 

 ings of numbers of individuals of the several species made to the 

 same scale. It appears at once that T. depressa and T. triincata 

 are small species, that T. hicornis, T. amhulatrix, and T. exilis 

 belong to a series of medium sized species, and that T. ramici- 

 formis and T. longicornis are the largest ones. Variations in the 

 length of the antapicals and of the neck are the principal cause 

 of fluctuations in the size. In the cases T. hicornis, T. trnncata, 

 T. ramiciformis, and 2\ longicornis where a number of individ- 

 uals have been measured, the extreme measurements are usually 

 relatively rare and most of the individuals lie very near the mean. 

 There is thus a norm in the matter of size for each species. 



It comes frequently to he a matter of experience to one who 

 examines many organisms with a view to assorting them accord- 

 ing to their kinds, that differences are detected en mass, without 

 analysis of the individual differentia Is which together constitute 

 the facies or ensemble of characters which is diagnostic of the 

 species. In a relatively simple organism such as Triposolenia 

 these individual structural characteristics of the species may be 

 readily isolated and a comparison of their assemblages and rela- 

 tions affords some data bearing on the nature and origin of 

 species. 



The accompanying table of species characters of the eight 

 known species in this genus has been drawn up to include those 

 characters which are most easily defined or simply expressed. 

 Others, principally in the more complex matter of proportions of 

 various parts, might be added to the list. 



It might be supposed that in so simple an organism as Tnpo- 

 solcnia species might be separated by single characters. An 

 examination of the table and the plates will suffice to show that 

 this is decidedly not the case. For example, T. trnncata differs 

 from its nearest relatives, T. hicornis and T. depressa in size, 

 shape of head and epitheca, lengtJi, curvature, and angle of the 

 neck, proportions of the anterior process, shape of the midhody 

 (from T. depressa only), proportions, curvature, and spread of 

 tJie antapicals as well as in their tips. There is, indeed, scarcely 

 an element of the organization which on close examination does 



