The Pteropods are not largely represented in fossiliferous deposits, but they have a wide range in time, extending from the Upper Cambrian rocks up to the present day. The Theca and Conularia of the Palaeozoic period, if truly Pteropods, are of comparatively gigantic size. Both commence their existence in the Silurian or Upper Cambrian, and the former is entirely Palaeozoic. The genus Conularia, however, extends into the Mesozoic period, and is found in the Liassic rocks. The Silurian fossils which form the genus Tentaculites, though often referred to the Tubicolar Annelides, appear to belong without doubt to the Pteropoda. The recent genera Hyalea (fig. 223), Cleodora, and Cuvieria are represented in the Tertiary period.