The triangular shape shell of Cyrtina is very easy to spot. It is a prolific member of many Devonian communities and is one of the more common fossils found. The specimen below is Cyrtina hamiltonensis and was found in the Silica Shale (Devonian, Givetian stage) at Milan, MI.
Some other examples of Cyrtina that I've blogged about are Cyrtina umbonata from Michigan, Cyrtina sp. from Iowa, Cyrtina arkonensis from Ontario, other Cyrtina species from Ontario.
Allow me a moment to explain my thought process behind this blog. The photo of the pedicle valve shown above was taken with far end of the pedicle valve in focus and so
the tip closest to the camera is out of focus. Still, it gives you a
view similar to those you find in older reference books with hand drawn
illustrations of fossils. Imagine how frustrating it would be to try and
ID your fossil when they only give you a view of the pedicle valve and brachial valve similar to my above pictures. This is part of the reason why I include anterior, posterior and profile views in my posts.