DORIDOEIDES GARDINERI. 295 
Nudibranchs is perhaps explicable by the fact that the larvee 
of the latter reject the shell before the gill is formed. 
Now the absence of gills is certainly not a primitive 
condition in mollusca, and in many groups the gill-less forms 
are obviously specialized or degenerate. But the Nudi- 
branchiata are admittedly derived from the Tectibranchiata 
by suppression of the ctenidium and, as parallel forms are 
found with and without secondary gills among the less 
specialized nudibranchs (e.g., Tritonia and Tritonidoxa), 
it may be that the gill-less forms remain as a record of the 
first weak effort to develop a new type which greatly 
increased in strength and variety by the acquisition of 
secondary branchiz. Tritonia and Doris are clearly much 
more successful types than Tritonidoxa and Doridoxa 
and their superior respiratory apparatus may be the cause, 
On the other hand if the forms with gills are supposed to be 
the earlier, it is not obvious why so many families have lost 
their gills. The special conditions of pelagic and fossorial 
life might explain their disappearance in Phyllirrhoe and 
Pleuroleura (though Pleurophyllidia which has bran- 
chial lamellze and is much richer in species than Pleuroleura, 
is also fossorial), but there is no obvious specialization about 
the other forms. Tethys (with pallial branchie) and Melibe 
(without them) are very similar forms, and in some respects 
Melibe seems the more archaic of the pair, since it possesses 
jaws which Tethys has lost. The question can be settled 
definitely only by the discovery of forms more primitive 
than those now known (that is to say, clearly intermediate 
between the Tritoniide and the Tectibranchs), and we 
merely wish to indicate the shape it assumes in the light of 
the interesting new genera recently discovered. Tritoni- 
doxa and Tritoniella are little more than Tritonias 
without branchiz: Doridoxa is a real connecting link, a 
Tritonid with many of the special characters of the Doridide. 
Dirona is a Tritonid with papille on the back, but the liver 
though lobed is not ramified: Charcotia also seems to 
VOL. 592, PART 2,—NEW SERIES. 22 
