342 PROCEEDINGS OF THE ACADEMY OF [1896. 



penis has been lost, and the epiphaUus directly enters the atrium. 

 In these forms the vagina assumes the function of an evertable penis, 

 an extraordinary but by no means unparalleled instance of change 

 of function. 



These matters ai-e here dwelt upon somewhat fully, because in all 

 former American work on slug anatomy, no discrimination whatever 

 has been made between the penis and the epiphallus, the very real 

 and important morphologic fticts involved being, therefore, entirely 



Ignored. 



The most prominent general feature of the genitalia in the three 

 genera is the crowding of the main mass forward into the anterior 

 half of the body-cavity. 



GENERIC CHARACTERS. 



The three genera o^ Arioiddce mentioned above are seen by the 

 foregoing general description to present many common features in 

 their digestive and generative organs, showing them to be nearly 

 allied. Their main difierential characters are shown in the follow- 

 ing analysis: 



I, Respiratory pore anterior, the genital orifice below it. No 

 caudal mucus pore. Back rounded in adults. Stomach extend- 

 ing back of posterior loop of intestine. No penis, an epiphallus 

 replacing it ; ovotestis widely separated from the albumen gland, 

 situated in the cavity of tail, behind the stomach (see PI. XIII, 

 tig. 3, A. hortensis), Genus Arion Ferussac. 



11. Respiratory pore behind middle of shield. Genital orifice near 

 right tentacle. A caudal mucus pore. Back keeled, at least 

 toward the tail. Posterior loop of intestine behind stomach. 

 Ovotestis packed close to the base of albumen gland. 



a. No penis, a short epiphallus replacing it (see PL, XIV. fig. 

 12) ; right eye retractor passing to the left of genitalia. 

 Genus Aphallarion Pilsbry and Vanatta. 

 aa. A well developed penis, with short, fleshy retractor mus- 

 cle; epiphallus more or less introverted in penis (see PI. 

 XIV, tigs. 7, 8, 9, 14) ; right eye retractor passing between 

 $ and 9 branchesof genitalia. Genus ApioLiMAxMorch. 

 One species of the Paljearctic genus Arion has been introduced 

 by commerce within our limits, ^4. hortensis Fer. It occurs at Bos- 

 ton and New Bedford, Mass, ; Poughkeepsie, N. Y. ; Seattle, Wash., 

 etc. 



Genus ARIOLIMAX Morch. 



External Characters. — Body limaciform, its posterior half 

 more or less keeled on the back ; foot margin defined by deep pedal 



