ABERRANT FORMS OF LAMELLIBRANCHIATE GILLS, 607 
original one. Between the simple gills of Nwcula and most 
complex ones known, there are a great many intermediate 
stages, some going more in one direction, others in another. 
For instance, Lucena and Corbis are said to have only one 
gill-plate on each side (‘Owen’s Inverteb.’). According to 
Sars, Pecchiola is in the same condition (‘ Remarkable 
Forms of Animal Life,’ G. O. Sars). Chamostrea and Myo- 
chama are described by Hancock (‘ Ann. and Mag. of Nat. 
Hist.,’ 1852-3) as having the inner gill-plate complete, but 
the outer plate lacking the outer lamella. In these tentacular 
filaments seem to be fused with each other. On the other 
hand, although Arca, Mytilus, Modiola, have all the lamelle 
present, the filaments composing them have not fused with 
one another. It is interesting to notice that Nucula and 
Yoldia, in which the gills have remained rudimentary, 
have, as Dr. Brooks first pointed out to me, an unusual 
power of locomotion, while forms wholly or almost wholly 
unable to move, as Ostrea, Pholas, &c., possess highly- 
developed gills. 
For some reason the inner gill-plate seems to develop 
further than the outer. For instance, in many genera, the 
inner is much larger than the outer. In Chamostrea and 
Myochama, already referred to, it is the inner gill-plate that 
is complete, and the outer gill-plate that lacks a lamella. It 
will also be seen a little further on that in Anodon the inner 
gill-plate has gone further than the outer in its development. 
In the embryological study of the branchie of Mytilus 
Lacaze-Duthiers observed that the filaments of the inner 
gill budded out first. 
It is very instructive to see the process of secondary fold- 
ing going on in higher varieties of the gill. The two 
lamelle of a gill-plate are, in such a case, no longer parallel, 
but wavy, and the surface of a lamella is thus considerably 
increased. In Anodon this process is perhaps going on, for 
Peck shows that in that genus the cross-section of the outer 
gill-plate has parallel and straight edges, but that the outer 
lamella of the inner has a wavy margin. Posner shows 
successive stages of secondary folding in the gills of Pholas 
dactylus, Venus (sp), Mya arenaria, Ostrea edulis, Solen 
vagina, Cardium edule, Pinna nobilis. 
Diametrically opposite, as the views advocated by Posner 
and Peck may seem, it is not difficult to reconcile the two. 
If we look over the list of the genera examined by Posner, 
we shall find all of them, except Mytelus and perhaps Pecten, 
to possess more complex gills than Unio, and starting, as he 
did, from the last genus, it is no wonder that he considered 
