PILSBRF : ON METOSTRACON, N.GEN. 29 



the jaw is solid and arcuate with a few stout ribs, and the dentition 

 as in the Helices. There are no pedal furrows. The shell is strongly 

 depressed, with 1\ whorls, the apical one not differentiated by special 

 sculpture ; it is open below, the columella appearing as a mere raised 

 lamina along the sutures. It thus resembles some of the rounded 

 species of Haliotis, such as H. ovina, in form. It is very fragile, and 

 covered with a yellowish cuticle. Gabb's type-specimens, which are 

 the only ones known, have been dried, though preserved in alcohol. 

 They show the shell externally. Perhaps the mantle was not actually 

 closed over the shell, and is retracted. The internal anatomy is 

 unknown ; but I am somewhat disposed to think that it will be 

 found to agree in the main with Xanthonyx, or possibly Metostracon. 

 The extreme similarity of the shell to that of Peltella (Brazil) and 

 Gceotis (Porto Rico), both of which are bulimuline, render a decision 

 on the systematic position of Cryptodracon impossible, until at least 

 the genitalia have been examined. 



General Affinities of Metostracon and Xanthonyx. 



At first sight one is inclined to refer these genera to the Arionida?, 

 the external form recalling Binneya and Hemphillia. Closer inspection 

 shows that the resemblance is most superficial. The present genera 

 have no pedal grooves ; the folds of the alimentary canal are wholly 

 differently disposed ; the buccal retractor is united with the left ocular 

 band in Metostracon, but not so in Arionida?. The pallial organs differ 

 widely. The genitalia are wholly diverse, Metostracon and Xanthonyx 

 having an epiphallus and flagellum on the penis, and a dart-sac with 

 two well-developed mucous glands, while the penis in Arionida? never 

 has a flagellum, and a dart-sac and mucous glands are never developed. 



The Liinacidce and Philoniycidoe and the slug-like elasinognathous 

 genera Omalonyx and Neohyalimax are so obviously different from 

 Metostracon and Xanthonyx that no special comparison is necessary. 

 There remains only one family of American Yasopulmonata known to 

 have slugdike members, the Bulimulidae. The genera Amphibulima, 

 Pellicula, Gd'otis, and Peltella are more or less slug-shaped, but the 

 jaw and dentition in these groups are cleai'ly bulimuline, and the 

 genitalia, so far as known, are perfectly simple, as in Bulimulus and 

 Drymmis. 



No known group of slugs, or semi-slugs, will therefore receive 

 Xanthonyx and Metostracon ; but among the Helicida? we find that the 

 Belogona Euadenia, or Asiatic- Amei'ican group of clart-bearers, agrees 

 in all important characters. The Californian genus Epiphraymophora 

 and the Antillean Cepolis arc apparently the nearest allies of Meto- 

 stracon and Xanthonyx. The details of the genitalia are practically 

 identical. The free muscles are nearly the same. The external 

 characters of the foot, the jaw and dentition, the respective positions 

 of the gonad and liver, are all in agreement. The only features in 

 which Metostracon and Xanthonyx differ materially from the American 

 Belogona are those directly correlated with the degeneration of the 

 shell. The shell is reduced, the mantle modified in form, the kidney 

 and lung greatly shortened, and the position of the heart changed. 



