Journal 



OF THE 



NEW-YORK MICROSCOPICAL SOCIETY. 



Vol. VL JANUARY, 1890. No. 1. 



ON THE LINGUAL DENXmON AND SYSTEMATIt: 



posrnoN OF pyrgula. 



BY C. E. BEECHER. 

 {Read November \st, 1889.) 



The genus Pyrgula has been so often assigned to various 

 families of the mollusca by different authors, that it has had a 

 truly remarkable experience. There now seems to be about an 

 equal weight of opinion as to whether it should be placed in 

 the family Rissoidffi or among the Melaniidae. Without more 

 definite information than that expressed by the shell, it appears 

 merely a matter of choice as to which group should include the 

 genus. This condition is but one of many, in which development 

 in parallel lines produces forms having great similarity in 

 external appearance, and in which recourse must be had to 

 more important and reliable characters than external form, in 

 order to ascertain their true systematic position. 



True systematic data should, if possible, be based upon the 

 assemblage of all the characters of the organism, and it is 

 unsafe to take any one as infallible. In the present genus 

 — Pyrgula, and among many similar small turreted and globular 

 shells, as the Rissoidce, which commonly vary within narrow 

 limits, any important member of the animal which exhibits great 



Explanation of Plate 21. 



(o) Two members of the radula with the teeth in their normal positiou. 



(6) The teeth of the second or outer pleural series; showing their spoon-like form 



(c) The teeth of the first pleural series; carrying eighteen denticles each. 



(d) The lateral teeth; showing the long slender peduncle, the quadrate body 

 with an alveohis, and the curved serrula bearing ten denticles. 



(e) Two of the rachidian teeth; showing their form and the arrangement of the 

 denticles. 



All figures magnified 500 diameters. 



