98 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



and consequently more twisted ; but it differs in having a penis. In 

 this character, and the long cusps of the teeth, Anadenus resembles 

 the less specialized American genera Ariolimax and Helper avion. 



In brief, Anadenus is more closely allied to Prophysaon than to any 

 other known genus, but in the developed penis shows distinct affinity 

 to Sesperarion. Clearly belonging to the Arioninse, on account of 

 the specialized condition of the retractor system, it is in a more 

 primitive stage than any other known member of this subfamily. 



Genus GEOMALACIIS. 



The elaborate treatment of this genus by Simroth,^ Scharff,^ and 

 Godwin-Austen ^ leaves little to be desired. It is essential, however, 

 to correct one current error in the interpi-etation of the genitalia, 

 so that an apparent anomaly may be explained, and the true affinities 

 of the genus be made clearer. 



Those who have dissected Arion have noticed that the spermatheca 

 duct enters the atrium between the oviduct and the epiphallus, and 

 that it is provided with a retractor muscle. Now in Geomalacus the 

 atrium on the side where the epiphallus enters has been pulled out 

 into a long sack, carrying with it the epiphallus and spermatheca, 

 and simulating a penis so perfectly that it has generally been regarded 

 as a penis carrying the spermatheca, and its retractor has been called 

 a retractor of the penis ; whereas this supposed penis is wholly 

 secondary, being an elongated portion of the atrium, and its retractor 

 is the exact homologue of the retractor of the spermatheca in Arion, 

 the resemblance to a retractor of the penis being only one of analogy, 

 not of homology. It is inserted far behind the diaphi'agm, not upon 

 it or at its edge, as the true retractor of the penis is. In this genus, 

 therefore, part of the atrium acts as a penis. 



The objection to calling this secondary penial structure a penis 

 is twofold : first, that its purely secondary and recent origin would 

 be lost sight of ; and second, that it is not morphologically equivalent 

 to the original penis, because a greater portion of the atrium composes 

 it, that part normally bearing the spermatheca being added to the 

 portion primitively penial. It is, however, extremely interesting to 

 find a Pulmonate in which the primitive penis, inherited from Tecti- 

 branch ancestors, has been lost, doubtless by gradual shortening, 

 and in which the associated retractor mechanism has disappeared ; 

 whilst the penial function has been shifted to the atrium (as in some 

 Arions, and probably the ancestor of Geomalacus), or even the vagina 

 {Prophysaon) or oviduct {Arion) ; and a new " penis " has been formed 

 by the elongation of a portion of the atrium, with a new retractor 

 muscle, morphologically totally diverse from the original retractor of 

 the penis, appropriated from the female side of the genital system. 



^ " Die Nacktsdmecken der Portugiesiscli-Azorisclien Faiiua" : Nova Acta Acad. 

 Ctes. Leop. -Carol. Germ. Nat. Cur., torn. Ivi, p. 351. 



2 " The Shio-s of Ireland" : Trans. Eoy. Dublin Soc, ser. ii, vol. iv, p. 551. 



3 Laud and Fresli-water MoUusca of luclia, vol. i, pp. 60-G4, pi. xii, figs. 1-8. 



