38 A MONOGRAPH OF THE 



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. Tritoniad;e. Tentacula sheathed ; gills fusiform or brauched on each side of the back ; 

 vent lateral ; jaws horny. 



Tritonia, Dendronotus, Scyttcea, Eumenis. 

 Fam. 5. Proctonotidje. Tentacula simple, linear, not sheathed ; gills fusiform, on the sides of 

 the back ; vent dorsal ; jaws horny, strong. 



Proctonotus, Antiopa. 

 Fam. 6. DiPHYLLic-iADiE. Tentacula simple, united, expanded ? ; gills in folds on the under side of 

 the edge of the mantle, which is bent up ; jaws horny. 



Diphyllidia. 



b. Tongue narrow ; teeth in a single central series. 



Fam. 7. Dotonid;e. Tentacula sheathed at the base, retractile ; gills fusiform, on tbe sides of 

 the back. 



Doto. 



Fam. 8. GlaucidjE. Tentacula subulate, simple, rarely ringed, contractile; gills fusiform or 

 branched, on the sides of the back ; jaws often horny. 

 a. Glaucus j j3. Eolidia, Montagua, Favorinus ; y. Embletonia ; B. Hermcea ; e. Alderia. 



Fam. 9. Placobranchid^e. Tentacula subulate or linear, folded ; gills in the form of plaits or 

 vessels radiating on the surface of the back. 



Placobranchus, Elysia. 

 Fam. 10. LiMAPONTiADiE. Tentacula none or simple, contractile; body depressed; gills none 

 external. 



Limapontia. 



Fam. 11. Phyllirrhoid^e. Tentacula elongate-subulate; body compressed vertically; gills 



none external. 



Phyllirrhoe. 



c. Tongue and jaws none. 



Fam. 12. Phyllidiad^;. Tentacula dorsal, anterior, retractile ; labial palpi close, conical, small ; 



gills in form of radiating folds on the under side within the edges of the mantle ; vent 



medial, posterior. 



Phyllidia, Fryeria. 



The character derived from the dentition of the tongue, it will be seen, has been largely 

 used in the above arrangement. The value of the lingual apparatus in the classification of the 

 Mollusca has lately occupied much attention, but has scarcely yet been satisfactorily deter- 

 mined. Taken as a primary character, this organ would lead to the most arbitrary grouping 

 of dissimilar forms. As a secondary, it is more useful : but, still, subordinated as it has been 

 by Dr. Gray to the disposition of the branchiae, it appears to us to have been rather too much 

 relied upon in his arrangement. Its indications are uncertain. In some groups the lingual 

 spines will be found to vary very slightly throughout all the genera of a family ; while in 

 others, different kinds of tongue are associated in genera which from their other characters 

 evidently belong to the same family. This is frequently the case in the Nudibranchs as well 

 as in the Bullidce and some of their allies. Having paid great attention to the character 



