254 TKOCEIiDlNGS OF THE MALACOLOGICAL SOCIETY, 



ON THE SINISTRAL CUARACTER OF THE SHELL OF riANOEBIS. 

 By J. Henry Vanstone. 



Read FebruarijWi, 1895. 



The question as to whether the discoidal shell of Tlanorhis is dextral 

 or sinistral has occupied the attention of many naturalists from tins 

 time of Linniieus. Although within recent years the researches of 

 the late Dr. Paul Fischer fully confirmed the sinistral nature of tliis 

 shell, yet many writers still maintain the opposite theory, and 

 therefore a brief review may be acceptable to those interested in 

 this subject. 



Among the earliest writers wc find that Linnaeus, Miiller, and 

 Draparnaud considered the shell to be right-handed, whilst Cuvier 

 adopted the sinistral view, basing his opinion on the anatomy of the 

 animal. Somewhat later Des Moulins discussed the question, and 

 having shown on what grounds he believed the shell to be dextral, 

 went out of his way to prove the animal to be similarly coiled, in 

 spite of his knowledge of the position of the external orifices. 

 Previous to this Deshayes had formulated a law which stated that 

 the more dilated portion of the peristome always indicated the upper 

 side of a shell. This generalization, which holds good in shells of 

 elevated spires, would, in the absence of certain other facts, tend 

 to uphold the dextral character of Planorhis, and it is on Deshayes' 

 law, together with the position of the shell in the living animal, that 

 the dextral theory has been established. 



About the time of Des Moulins, Jacquemin published his memoir 

 on the development of Flamrbis, and his figures being in every case 

 reversed by mistake, the young shell is represented as right-handed. 

 This unfortunate error remaining for a long time unnoticed, simply 

 strengthened the opinion that the shell was dextral, as upheld by 

 Moquin-Tandon, Pire, and others. In 1852 jS'aumann souglit to 

 establish the sinistral side of the question on mathematical grounds, 

 a similar course, however, being taken thirty- eight years later by 

 Pfeffer, in support of the opposing view. 



On many occasions the embryos of Planorhis have been selected 

 for the study of Pulmonate development, but in nearly every instance 

 the observers have confined themselves to the changes prior to the 

 appearance of the shell. However, in 1877 Paul Fischer decided to 

 again examine the young shell of P. corneas and if possible prove its 

 sinistrosity. lleferring to the two authors who had figured tlie young 

 shell, he says: " Les figures de Quatrefages sont insufiisants pour 

 decider si les embryons sont dextres ou senestres ; celles de Jacquemin 

 montrent des embryons a coquille spirale dextre ; mais toutes les 

 planches de cet auteur sont fautives par la negligence du dessinateur, 

 qui a transporto les dessins sur pierre sans les retourner. C'est ainsi 

 qu'il represente un aniuud adulte de I'lanorbis avec ses orifices gcnitaux 



