No. 411.] HINGE TEETH OF BIVALVES. ISI 
with the completion of the crenulations which in many other 
pelecypods form only the nepionic hinge or provinculum. The 
provinculum is common to all the Prionodesmacea, bivalves 
which include Neumayr’s Taxodonta (Arca, Nucula, etc.) and 
Dysodonta (Mytilus), and other ancient types (Unio, Perna, 
etc.). A few of the Teleodesmacea (Heterodonta and Des- 
modonta of Neumayr) also retain traces of it. The marvelous 
thing in the development of the hinge is that in the vast 
majority of hinges, including such primitive types as Nucula, 
the hinge does not go on developing from the provinculum, but 
the latter stops short and, as it were, after an interval, a wholly 
new set of teeth, taxodont or other, begins to develop. In the 
taxodonts there is a multiplication of similar teeth, often more 
or less A-shaped, separated by a gap from the most adjacent 
of the provincular teeth, even when closely similar to them. 
The explanation of this discontinuity in development is the 
most interesting question to be solved in connection with the 
pelecypod hinge. In the Teleodesmacea instead of a multi- 
plication of similar teeth we have a few, usually with one limb, 
of the A disproportionately elongated. The apex of the A being 
adjacent to the umbo, the proximal portions develop into the 
so-called cardinals on the anterior side of the ligament. Only 
im forms (Crassinella) having a very symmetrical development 
is there any indication of a cardinal behind the ligament. The 
distal end of the long limb of the A, usually the upper or outer 
limb, develops into a so-called lateral tooth. This process is 
characteristic of the forms (Venus, Cyrena, Mactra, etc.) 
included by Bernard in his cyrenoid type of heterodonts. 
In others the connection between the processes called cardi- 
nals and laterals is practically wanting; the laterals arise 
separately from the first, as far as observation can determine 
at present. These are types having the hinge much elongated 
and the laterals distant (Tellina), or absent (Macoma), or in 
which the teeth are what I have called cyclodont (Cardium, 
Isocardia), arising from below the hinge margin and practi- 
cally without a hinge plate. Most of these were included by 
Bernard in his lucinoid type of heterodonts. There is nothing 
to deter us from believing that all these types are fundamentally 
