94 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



PHTLOGENT OF THE GENEEA OF ARIONIDiE. 



By Henry A. Piisbey, 



Conservator of the Conchological Section and Professor of Malacology 



in the Academy of Natural Sciences of Philadelphia. 



Read 13th May, 1898. 



PLATE VII. 



Ottr literature has been ■ enriched during the past decade by many 

 admirable papers upon slugs of the family Arionidse. In Germany 

 and England the study of Slugs has become a speciality, claiming 

 foremost place in the thoughts of some naturalists of large experience 

 and considerable attainments, and enlisting the co-operation of many 

 amateurs, without whom our investigations would be limited indeed. 



For all this, we have as yet had no general survey of the family ; 

 no indication of the affinities existing between the genera, and 

 consequently the reconstruction of their past history or phylogeny has 

 not been attempted; there has been no clearly expressed estimate of 

 the comparative value of their organic characters in classification ; ^ 

 and finally, no well-founded theory advanced accounting for the 

 present distribution of the group. 



With the data now accessible ^ it has seemed to me that the time 

 for safe generalization and sound classification has arrived ; the more 

 because we have little or nothing to expect from palgeontological 

 discoveries, this source of so much valuable information being prac- 

 tically denied us in dealing with the phylogeny of slugs. We must 

 depend solely upon comparative anatomy and embryology. The danger 

 of misinterpretation from the occurrence of convergent development 

 and other causes is great, and only to be guarded against by attention 

 to all the characters of the organism. 



For the primary division of the Arionidse the modifications of the 

 free muscles afford the most fundamental characters ; and their 

 arrangement also constitutes one of the most weighty differences 

 between Arionidae and other slugs. The free muscles of land snails 

 belong to two groups : ( 1 ) Ketractors of the foot, phaiynx, and 

 tentacles, and (2) retractors and other muscles of the genital organs. 

 Muscles of the first group are very constant in form and position, 

 those of the second group quite variable. The principal muscles are 

 represented on PI. VII, Figs. 1 , 4, and are as follows : — Pharyngeal 



^ The general tendency haa been to attach too much importance to the modi- 

 fications of tlie genitalia. While these organs afford excellent specilic and generic 

 characters, they are of minor value in the larger classification of this family. 



^ The only genera upon which information essential to this inquiry is lacking, are 

 Letourneuxia (Algeria), Anadenulus (California), and Cryptostracon (Costa Hica). 

 The last-mentioned may belong elsewhere. 



