?>Q PROCEEDINGS OF THE MALACOEOGICAL SOCIETY. 



In the muscular system, the foot retractors are absent from the ocular 

 bands, and there is no tail retractor. 



While these numerous differences are all consequent upon the 

 reduction of the shell, and would inevitably follow such reduction, 

 one is somewhat inclined to group the genera Xanthonyx, Cryptostracon, 

 and Metostracon together as a subfamily. This course would, however, 

 be premature, as it is by no means certain that the three genera 

 originated from one and the same genus of normal Helicoid Belogona ; 

 the different way in which the displaced liver, kidney, heart, anil 

 ovotestis have been rearranged in their more confined space in the 

 several forms suggesting parallelism rather than a common phylogeny. 



EXPLANATION OF PLATE III. 



Metostracon mima. 



Fig. 1. Outline of exterior, right side, specimen from Uruaparn. The peni^ is 



exserted. 

 ,, 2. Kidney, from below, the liver removed, its outline and that of the 



concealed heart represented by dotted lines. Cf. Fig. 4. 

 ,, 3. Kidney, from above, its attachment to roof of lung shaded ; heart visible 



in the sinus between anterior and posterior lobes. 

 ,, 4. Organs of visceral dome and free retractor muscles, from below. 

 ,, 5. Shell of type-specimen, from Morelia, Mexico ; upper surface. 

 ,, 6. Shell of specimen from Uniapam ; ventral surface. 

 ,, 7. Apical end of same, much enlarged, showing columellar ledge across the 



spire-cavity. 

 ,, 8. Genitalia of type-specimen. 

 ,, 9. Jaw of type-specimen. 



,, 10. Median, adjacent lateral, and a marginal tooth of type-specimen. 

 , , 11. Digestive tract, from above. (Diagrammatic.) 



Xanthonyx Salleanus. 



,, 12. Animal from right side, An alcoholic specimen (collected at Texolo, 



State of Veracruz, Mexico, by S. N. Khoads). 

 ,, 13. Pallial region'. 

 ,, 14. Genitalia. 

 ,, 15. Free retractor muscles. The restoration of the pharyngeal retractor is 



hypothetical, and the complete separation of the posterior ends of the 



tentacular retractors is uncertain. 



a.gl. Albumen gland. l.t.r. Left tentacular retractor. 



col. Columellar muscle. m.gl. Mucous glands. 



d.s. Dart-sac. o.t. Ovotestis. 



cpi. Epiphallus. p. Penis. 



//. Flarjellum. p.r. Retractor of the penis. 



G 1 , G-, } Successive folds of the in- ph. Pharynx. 



G 3 , G K . ) testiual tract. pJi.r. Pharyngeal retractor. 



h. Ileart. r.o. Right ocular tentacle. 



Jul. Hermaphrodite duet. r.t.r. Eight tentacular retractor. 



/,-. Kidney. sp. Spermatheca. 



L. Liver. vag. Vagina. 

 l.o. Left ocular tentacle. 



