58 ONCHIDORIDIDiE. 



cavity, although they are arranged in a circle round the vent 

 on the hinder part of the mantle. The tropical species 

 exhibit considerable activity, and by their splendid colours 

 and varied markings give an air of great vivacity to the 

 groups of corals and banks of sea- weed among which they 

 take up their abode. 



Genus ONCIIIDOHIS, Blainville. 



Tentacles dorsal, retractile. Mantle strengthened with 

 spicula. Gills each retractile into its proper cavity. 



Syn. Onchidorus, Ferus. Onchidora, Guv. Onchi- 

 diodoris, Ayassiz. 



Ex. 0. pusilla, Alder and Hancock, pi. 63, fig. 7. 



The tentacles for a considerable part of their length are 

 surrounded by elevated rings ; the mantle does not always 

 entirely cover the foot, and its surface is nodulose, the no- 

 dules being interspersed with numerous calcareous spicula. 



Species of Onchidoris. 



depressa, Alder and Heme. pusilla, Alder and Hanc. 



inconspicua, A IderandHanc. sanguinea, D'Orb. 



Leachii, Blainv. sparsa, Alder and Hanc. 

 muricata, Mull. 



Genus VILLIERSIA, D'Orbigny. 



Mantle forming a continuous calcareous shield, pierced 

 with holes for the tentacles, gills, and vent. Gills in the form of 

 two lobes, separate from each other and from the anal orifice. 



Ex. V. scutigera, D'Orbigny, pi. 63, fig. 8. 



In this very singular genus the tentacles are retractile and 



