of the Nudibranch Mollusks. 319 



first, the necessity of dividing the family Dorida into three fami- 

 hes, neatly characterized by the disposition and form of the 

 teeth, as well as by the position of the respiratory organs,— cha- 

 racters showing very great differences m their habits and oeco- 

 nomy ; secondly, proving that the genera Phyllirrhoe and Lima- 

 pontia, though they have no external gills, or indeed any ap- 

 pearance of gills of any kind, are properly placed in this order, 

 as the structure of the buccal mass, tongue, and teeth exactly 

 agrees with that of the families next to which they were placed 

 in my previous arrangement. 



I may observe, that I find the disposition and form ot the 

 teeth to afford a very good guide in cases which have been con- 

 sidered doubtful from some modification of the respiratory or- 

 gans The genera Ancylus, Siphonaria, and Amphibola have 

 been placed in different parts of the system, even m the most 

 recent work of Philip pi— often with marine families. From the 

 structure of their respiratory organs, I had long satisfied myselt 

 that thev are all true Pulmonohranchia, and the examination of 

 the tongue and teeth strengthens this opinion, for it is almost 

 impossible to distinguish their teeth from those of the Aun- 

 culada and other land Mollusca. 



In the following table, the teeth of the genera placed under 

 each family have been actually examined, or their teeth have 

 been described or figured by some other author. 



1. Gills surrounding the vent, on the middle of the hinder part 

 of the back. 



Fam. 1. Onchidokid^. Teeth two in each cross series; gills 

 in separate cavities ; mantle edging the foot and simple. Acan- 

 thodoris, Onchidoris. 



Fam. 2. DoRiDiD^=Doridina and Polycerina, Gray. Teeth 

 many in each cross series, subsimilar, inner often smaller; gill 

 iu a common cavity ; mantle edge simple. «. Dons; /3. Gonio- 

 doris, Ceratosoma ; 7. jEgires. 



Fam. 3. Triopid^ = Triopina, Gray. Teeth many (rarely 

 only four) in each cross series, the inner lateral ones large, irre- 

 gular-shaped ; gills in a common cavity; mantle small, edged 

 with tentacles. Triopa, Idalia. 



2. Gills superficial, generally in the form of fusiform processes, 

 plaits, or branching vessels. 



a. Tongue broad; teeth many in each cross series. 

 Fam. 4. TRiTONiADiE. Tentacula sheathed ; gills fusiform or 

 branched on each side of the back; vent lateral; jaws horny. 

 Tritonia, Dendronotus, Scyllaa, Eumenis. 



