2 JOURNAL OF THE [January, 



differentiation and is more diversified than the shell, naturally 

 affords a more convenient and trustworthy basis for systematic 

 determination. 



Too great reliance may be often placed upon trivial differ- 

 ences in the lingual dentition, but it is believed that within 

 reasonable limits, the characters of the odontophore are 

 extremely serviceable for purjDoses of classification. Especially 

 is this true for family distinctions, and on this account, such 

 features are commonly recognized as of much, though not of 

 sole, importance 



The radula in the Rissoidfe is very characteristic, its princi- 

 pal diagnostic feature being the presence of basal denticles on 

 the rachidian teeth. The accompanying illustration and 

 description of this organ in Fyrgiila annithita, C. «& Jan. 

 {P. helvetica, Mich.) the type species of the genus, show that 

 basal denticles are not present on the rachidian tooth, and, 

 therefore, it cannot properly be arranged with the Rissoidai, 

 but is related to the Melaniidae. 



The genus is at present unknown in North America, as the 

 species which were originally ascribed to it have since been placed 

 in other genera. The first American species thus referred was 

 Fyrgithiscalarifoniiis, Wolf. (Am. Jour. Conch., vol. v. p. 198, pi. 

 17? f- 3» 1S70) from the Post Pliocene of the Illinois river. This form 

 is now believed to belong to the genus Pyrgulopsis, Call and 

 Pilsbry.' In 1883, R. E. C. Stearns described a shell from 

 Pyramid Lake, Nevada, and placed it in Pyrgula {P. nevadensis). 

 The dentition of this form was described and figured by the 

 writer in 1884,'' but as, at that time, the true dentition of 

 Pyrgula was unknown, no comparisons could be drawn, and the 

 systematic position of the species remained unchallenged. 

 Subsequently Call and Pilsbry {Joe. cit) proposed the genus 

 Pyrgulopsis for this and allied species based upon conchologic 

 features. The only difference in the shell noted is that Pyrgula 

 is bicarinate or multicarinate, while Pyrgulopsis is characterized 

 as a unicarinate form. 



'On Pyrgulopsis, anew geuus of Rissoid mollusk, with descriptions of two new 

 forms. Proc. Davenport, Acad. Nat. Sci., Vol. V. 1886. 



^Call and .Beecher. Notes on a Nevada shell iFyryiUa lie vade/is/s). American 

 Naturalist, vol. xviii., pp. 853, 854. 1884. 



