To improve my knowledge of bivalves, I decided to approach the problem systematically. Rather than identify random specimens from the beach, I thought I'd get out my copy of Mollusca: The Southern Synthesis and work through the groups one by one.
Bivalves, according to the book, are divided into five subclasses. (The authors acknowledge that this is a bit of a dodgy classification but it's one we can work with.) So, I started reading about the first one — Protobranchia.
Protobranchia is defined by the structure of the gills … which aren't present in empty shells on the beach. Bugrit! Millennium hand and shrimp. Oh, yes, there's some alternative arrangement that relies on hinge teeth — one group of protobranchs has a particular array whereas the other doesn't — but other taxa also have ...
I moved on.
The next subclass in the book is Pteriomorphia. This includes scallops, oysters and mussels (but not cockles). And you can identify them from shells even when they're not alive, alive-o.
My rudimentary knowledge of bivalve anatomy carried me through the references to the heteromyarian and monomyarian conditions, which relate to the number and size of the muscles that hold the shell closed. (They are represented by oval or circular scars on the inside of the shell.) But then I got to this bit ...
Ligament type is variable within the subclass. The ligament is elongate, parivincular and opisthodetic in heteromyarian forms, such as mytiloids and pinnoids, duplivincular in arcoids and some pteroids, multivincular in isognomonids, and alivincular and amphidetic in equilateral monomyarians such as ostreids and pectinids.
… and my mind said to me, "When I grow up, I'm going to Bovine University."
So I might take this a little more slowly. I'll keep you informed.