120 University of California Publications. [Zoology 



not exhibit a difference in the species in question. These differ- 

 ences are not so pronounced between T. bicornis and T. deprcssa 

 as between T. exilis and T. romiciformis, nevertheless it is evi- 

 dent that even these more nearly related couples are distinguished 

 at once by a complex of characters rather than a single one or 

 even a few. The differences between T. exilis and T. ramici- 

 formis are discussed under the description of the former. The 

 differences between T. bicornis and T. clepressa are in size, form 

 of midbody, proportions of neck and of cytopharyngeal region, 

 and in the tips of the antapicals. The elementary characters of 

 the organism are thus fundamentally and extensively different in 

 their form and relations in the different species in this genus. It 

 is as though they had been thoroughly shaken up and readjusted 

 in the case of each species, when nature cast the die. 



The individual species characters are all subject to variations 

 of the fluctuating kind, in varying- degrees of amplitude. Thus 

 the length of neck and the convexity of the margins of the mid- 

 body are cpiite variable. Variation is least evident in the ante- 

 rior process, and the head. The greatest variation is perhaps 

 found in the antapicals, in their length, in the spread, degree, and 

 localization of their curvature, in the number and distribution 

 of the tubercles and in the form of their tips. Triposolenia is 

 probably a divergent member of the Binophysidae, and it may 

 be that the antapical horns, especially the dorsal one, are phylog- 

 enetically recent structures. They are also peripheral in location, 

 though no more so than the head in Avhich fluctuating variations 

 are less evident. 



These fluctuating variations often cause an individual of one 

 species to approach another in some one or even several charac- 

 ters, as, for example, T. truncata falls within the size limits of 

 both T. deprcssa and T. bicornis, but approach in size is not ac- 

 companied by a proportionate change in the same direction in 

 all other differential characters. Again the antapical tips of 

 T. (Icpressa are sometimes partially truncated, but there is no 

 coincident fluctuation in the pitting of the thecal wall or flat- 

 tening of the epitheca characteristic of T. truncata; in other 

 words, variation is not total in the direction of a given species. 



