Fossil specimen of the nautiloid cephalopod *Casteroceras alternatum* from the Devonian Marcellus Shale (Cherry Valley Limestone Member) of Madison County, New York (PRI 5363). Specimen is part of the type and figured collections of the [Paleontological Research Institution](http://www.priweb.org), Ithaca, New York. Length of specimen is approximately 25 cm. Model by Emily Hauf.