These curious fossils were first detected in the quarries at Glammis, by Sir Charles Lyell, and from their striking resemblance to the cephalic shield of certain trilobites, were supposed, for a time, to belong to the class of crustaceans. The beds of Carmylie and Balruddery, yield these organisms in the greatest abundance. One solitary specimen, a fragment of two inches in length, of the smaller scaly net-mesh, has been obtained by me in the gray rock, on the south bank of the Tay. The heads are uniformly in the best state of preservation; indeed hundreds of these lie entire, where no part of the body has left the trace of an impression. M. Agassiz assigns, as the reason of this, the great difference that exists in the structure of these two parts, and especially in the disproportion of their dimensions and forms, which would offer a distinct resistance to the pressure to which the animals must have been exposed. “If, on the other hand,” he adds, “the heads usually present their superior surface to us, it is because their inferior surface, the cavity of the mouth, the branchial arch and sinuosities of the inferior bones of the cranium, are points of support comparatively more solid, and more adapted for sustaining the matter which has filtered into them, than a larger surface slightly convex, which would naturally be detached from the rock wherever a separation was found in it.” THE DEN OF BALRUDDERY presents us with a group of very remarkable fossils, comprising, in an area of the gray sandstone of a few square yards, innumerable impressions of the plant-markings already noticed, multitudes of the Cephalaspis, spines, and other ichthyolites, along with two entirely new genera of fishes of the order of Placoids. The sandstone here is of a very slaty character, splitting up into thin layers, betwixt every one of which some organism or other has impressed its form; and the different kinds are often so promiscuously huddled together, as to suggest the idea of some violent commotion in the element which collected and destroyed them. In the “Synoptical Table of British Fossil Fishes,” by M. Agassiz, we find inserted a _Parexus recurvus_, and a _Clematius reticulatus_, from this locality; they are represented simply as ichthyodorulites, no complete specimens of the creatures having been presented to him, nor indeed have any been as yet obtained. One of the specimens in the Balruddery collection, when returned by M. Agassiz, was labeled as a _Palæocarcinus alatus_: and in the 14th Livraison of his “Poisson Fossiles,” he thus writes:—“Enfin j’en dois aussi plusieurs espèces à M. Webster de Balruddery. Parmi ses échantillons j’en ai trouvé plusieurs d’un grand intérêt, parce qu’ils m’ont fait connaître que le genre Pterygotus que j’avais établi, il y a plusieurs années, sur des fragmens très-imparfaits, n’appartient point à la classe de poissons, mais bien en celle des crustaces. Une pareille erreur semble