. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Cancellated structure from shell of Hippurite, as seen in transverse section. Magnified 5 diameters. diploe has been described by Mr. J. E. Gray* as existing between the laminae of Ostren piirpurca; but in no other shells of existing Mollusca has any approach to it been yet discovered. A very regular cancellated structure, however, is exhibited in the singular extinct group of Rud'mtcs, where it makes up nearly the entire thickness of the shell (fig. 421.). The cancelli are usually short hexa- gonal prisms, terminated at
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Cancellated structure from shell of Hippurite, as seen in transverse section. Magnified 5 diameters. diploe has been described by Mr. J. E. Gray* as existing between the laminae of Ostren piirpurca; but in no other shells of existing Mollusca has any approach to it been yet discovered. A very regular cancellated structure, however, is exhibited in the singular extinct group of Rud'mtcs, where it makes up nearly the entire thickness of the shell (fig. 421.). The cancelli are usually short hexa- gonal prisms, terminated at each end by a flat partition ; consequently, a section taken in one direction (jig. 421.) will exhibit the walls of the chambers disposed in a hexagonal net- work ; whilst a section that passes at right angles to this will bring into view the trans- * Mngazine of Zoology and Botany, vol. ii. p. Cancellated structure from the shell of Hippurite, as seen in vertical section. Magnified 5 diameters. tion ; which might lead to the belief that, like the cells of the Pinna, they were so consolidated in the living state. But they are also to be met with entirely empty, or with their walls merely lined by calcareous crystals ; so that there can be no doubt that they were originally hollow. The presence of this structure assists in determining the zoological position of the curious group in question, which many considerations would lead us to regard as having been interme- diate between the Bivalve Mollusca and the sessile Cirrhopoda. And it may be added that, by the same evidence, the place of the curious Plewrorhyncus fiibernicus, a fossil which has been assigned to a different tribe by almost every naturalist who has examined it, would unhesitatingly be determined as amongst the Rudistes. Echinodermata.— The structure of the skeleton in this class is entirely different from that which we have found to be cha- racteristic of the Mollusca ; whilst, in its essential features, i
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