PILSBEY : PHTLOGENT OF AEIONIDiE. 99 



I would only add here that the musculature of the tentacles and 

 pharynx, as well as of the intestine, is, according to Simroth, 

 typically that of Arion, the buccal retractor being inserted well 

 behind the diaphragm as it is in Arion, but in no other genus of 

 the family. 



Genus LETOURNEUXIA. 



This Algerian genus, or subgenus of Arion, still awaits anatomic 

 chai-acterization. It seems to diiler from Arion in the strong, thick 

 ^' liinacelle," and the peculiar tail, which according to Bourguignat 

 has no caudal gland (doubtful !), but the integument, posteriorly, is 

 laterally expanded, free along the pedal grooves, and overhangs the 

 sole on each side. If the type of Z. Numidioa be normal and not 

 pathologic, these sppcial features give the slug generic rank, even 

 though the genitalia may prove to be like those of Arion. 



CLASSIFICATION. 



A profound student of the slugs wrote in 1885 of Arion as follows : 

 "Die Grattung hat niemals eine Schale gehabt, niemals eine hohere 

 Stufe der Beschalung erreicht, als sie Jetzt hat .... Arion tragt 

 alle Merkmale urspriinglichen Unbeschaligkeit an sich .... 

 seine Yorfahren aus anderen Wurzel dem Wasser entsprossen." ^ This 

 startling conclusion was based largely upon the fact that in Arion the 

 system of retractor muscles bears but little resemblance to that of 

 the snails, or such slugs as Limax, ITrocydus, or the numerous other 

 slug and semi-slug derivatives of the Zonitidae. The concentration 

 into one or two main muscle bands posteriorly has been quite lost 

 in Arion and its immediate allies. 



When the Ariolimacine genera are taken into account, however, 

 the anomalous myology of Arion is explained; and that genus takes 

 its place as the terminal member of a series of forms beginning with 

 Binneya, half slug, half snail, with nearly the musculature of the 

 snails, and passing by numerous intermediate stages still existing, to 

 the abnormal Arion type. As a whole, the Arionidse not only do not 

 possess the characters of primitive shell-less forms, '■'■ Unheschaligkeit,''^ 

 but the series of recent genera unmistakably indicates their descent 

 from a group with well-developed spiral shell. It is to the Endo- 

 dontidse, a group which earlier had thrown off the slug-family 

 Philomycidte, that we look for the ancestral root of the Arionidse ; 

 just as the Limacidte came from Zonitid stock,^ and all form a 

 primitive Aulacopod branch. 



The direction of evolution in the Arionidse has been mainly (1) from 

 forms with the tail solid, as in snails, to those with it excavated and 



1 Dr. H. Simroth : Zeitsclir. f. Wissensch. ZooL, Bd. xlii, pp. 251, 252. 



2 These latter statements stand at present unsupported, because while the evidence 

 is largely worked out, I have not yet had time to elaborate it for publication. 



