PLEURODISCIDAE. 177 



to me a secondary and superficial one, as the former does not 

 share the essential character of Strohilops, which is the pos- 

 session of a system of internal laminae from a very early stage 

 to the adult. 



Pagodulina (Vol. 27, p. 166) has not been dissected by any 

 recent investigator. The figure of Moquin-Tandon, as far as 

 it goes, agrees with Orcula in form of penis and appendix, as 

 pointed out by Steenberg (1925). 



Family PLEURODISCIDAE. 



Pleurodiscidae Wenz, 1923, Fossilium Catalogus, Pars 21, 

 p. 1069. 



Patulastridae (Pleurodiscidae) Steenberg, 1925, Vidensk. 

 Medd. Dansk. Naturh. Foren. i Kobenh. vol. 185, 202. 



The shell is umbilicate, discoidal or turbinate, closely cos- 

 tulate or striate, the peristome simple and sharp. 



The viviparous animal has simple male end organs, the 

 passage of penis into epiphallus marked by an abrupt con- 

 traction, the simple penial retractor inserted on epiphallus; 

 no appendix or flagellum. The spermathecal duct is of 

 medium length (Pleurodiscus) or short {Pyramidula) , and 

 not forked. Jaw and teeth substantially as in Valloniidae. 



Two Palaearctic genera grouped together in this family are 

 not closely related, but they agree in the simplicity of the male 

 organs, and are not strikingly unlike in the shells. To include 

 them in the Valloniidae would destroy the reasonably homo- 

 geneous character of that group. 



Genera of Pupillidae without penial accessories have been 

 regarded as secondarily simplified end-products of evolution, 

 on account of their close relationship by the shell and other 

 qualities with genitally complicated genera (this vol., Intro- 

 duction), as in the case of Vertigo and Nesopupa. I can not 

 see any stem in the Valloniidae which will serve as a plausible 

 support for the attachment of Pleurodiscus and Pyramidula 

 thereto as simplified branches. 



Dr. H. B. Baker suggests that the Pyramidulinae and 

 Pleurodiscinae may represent primitive conditions, before the 

 evolution of complex genitalia (this vol., p. 193). In that 



