ORTMANN: FAMILIES AND GENERA OF NAJADES. 227 
which form there for this purpose. This ‘“‘unnatural’’ discharge is 
known only in the Lampsiline, and is unknown in the Anodontine and 
Unionine. Finally the family of the Unionide differs from the 
Margaritanide and is more highly advanced in the formation of rudi- 
mentary siphons. But in this respect this family is not very progres- 
sive. It has the anal and branchial openings separated only by the 
(complete) gill-diaphragm, and in addition, it has the anal closed above, 
thus giving it an incomplete tubular shape. Beyond this, there is no 
progress in this family. The presence of a supra-anal opening is, in 
my opinion, only incidental to the closing of the anal. 
The members of the third family, the Mutelid@, have gone in another 
direction in their development. If the expression may be permitted, 
they lay chief stress upon the better development of the siphons, while 
in the differentiation of the gill-functions they have started out from 
the beginning with another idea, which, however, has not attained a 
very high degree of perfection. With regard to this it may be said 
that they have restricted the marsupial function to the inner gills, 
and very likely the anterior connection of these gills with the palpi is 
incidental to this function. Not much advance is to be observed in 
the gill structure, and only two types are met: incomplete septa and 
intercommunicating water-tubes (a rather primitive condition) in 
one group (Hyriine); and complete septa and water-tubes in the other 
group (Muteline). Very likely the latter structures are not homolo- 
gous to the septa and water-tubes of the Unionide, but have been ac- 
quired independently, since their finer structure is different. With 
regard to the siphons, which attain within this family their highest 
perfection among the Najades, we have first of all a complete separa- 
tion of anal and branchial openings by a firm mantle-connection, 
which forms the posterior continuation of the gill-diaphragm, and in 
addition we have a tendency to close both the anal above, and the 
branchial below, by mantle connections. It is true that this tendency 
is not yet perfect in many Mutelide, but it is developed within this 
family, so that in the most highly specialized genera we have two real: 
tubular siphons, formed by complete coalescence of the edges of the 
mantle. 
I think the above account of the phylogenetic tendencies within 
the various divisions of the Najades will make it clear that the mor- 
phological characters upon which our new system is founded are 
