190 PLEUROBRANCHID^E. 



and of a faience white in front, is marbled behind by irregularly 

 disposed interrupted and entangled orange bands. In certain in- 

 dividuals some of these bands are a more or less deep brown. The 

 edges of the shell, very thin, sharp and lamellose, are of such 

 fragility that none of the specimens have it completely intact. 



Japan. 



The substance of Jousseaume's description is given above. He 

 examined four specimens. 



Family PLEUROBKANCHID^. 



Tectibranchiata Notaspidea in which the gill-plume arises about 

 the middle of the right side and extends backward, the dorsal shield 

 is fleshy, stiffened by spicules, and either shell-less, or concealing 

 wholly or mainly a delicate Haliotiform shell, the radula is multi- 

 serial, without rachidian teeth, and the jaws are well developed, com- 

 posed of many oblong plates arranged in tessellated pattern ; rhino- 

 phores present. 



The group is allied to the Umbraeulidce, but differs externally in 

 the want of a patelliform shell, the posterior gill, etc. It is not 

 closely allied to Runcinidce, which may be considered a specialized 

 and ancient branch of the Notaspidia, divergent in its reduction of 

 the radula to a triserial arrangement, and the loss of rhinophores 

 by degeneration. 



Pleurobranchidce are world-wide in distribution in tropical and 

 temperate (rarely in cold) seas. There are many species, especially 

 of the genus Pleurobranchus ; and the really diagnostic characters 

 of a large number of them are not yet known. 



Attention should be directed especially to certain characters, 

 which have been very generally neglected, but are essential to any 

 thorough systematic knowledge of species of the group. These 

 features are the following : (1) Positions and space-relations of the 

 genital orifices. (2) Number of leaflets or plumules of gill, length 

 of its adnate and free portions, and smooth or tuberculate nature of 

 the rachis. (3) Shape and denticulation of the individual plates 

 of the jaws. (4) Denticulation of the teeth of the radula. 



Characters (3) and (4) are readily observed with low powers of 

 the microscope ; no delicate manipulation being called for. With 

 adequate knowledge of the above-mentioned points, and with what 

 is already known of the animals, there would be but little difficulty 

 in constructing " keys " or tables for the easy identification of spec- 



