In looking over the Woodwardian Museum of the University of Cambridge last year, I perceived that Professor M'Coy had, among his many skilful arrangements, discovered that a fossil from the Lower Ludlow rock, which if detached I should have called an ichthyodorulite, was-one finger of the claw of a large crustacean analogous to the Pterygotus of Agassiz; an animal which, though at first classed with fishes, was long ago removed by that author himself into the order of Crustacea. In this case indeed there could be no mistake, for in the specimen, which Professor M'Coy had then named Pterygotus leptodactylus , one solid finger was seen to be closely attached at its base to an unequivocal cast of the other, thus representing the claw of a small lobster. Now although this body was taken, as above stated, from the Lower Ludlow rock, it appeared to be very desirable that Professor M'Coy should closely scrutinize some of the remains of the bone-bed of the Upper Ludlow which had been referred by Agassiz to fishes. For, as the great ichthyologist had formed his opinion from the drawings only which I sent to him, and had never had an opportunity of looking into the framework of those fossils with the microscope, it seemed not unlikely that some of them might also prove to be crustaceans. I accordingly procured for the examination of Professor M'Coy the specimens from the museum of the Geological Society which I had originally placed there, and others from the cabinet