Provided with this evidence, it may be now profitable to enumerate other correspondences, which are not perhaps in themselves inexplicable by Natural Selection, but which are more readily to be explained by the action of the unknown law or laws referred to--which action, as its necessity has been demonstrated in one case, becomes _a priori_ probable in the others. [Illustration: SKELETON OF AN ICHTHYOSAURUS.] Thus the great oceanic Mammalia--the whales--show striking resemblances to those prodigious, extinct, marine reptiles, the Ichthyosauria, and this not only in structures readily referable to similarity of habit, but in such matters as greatly elongated premaxillary bones, together with the concealment of certain bones of the skull by other cranial bones. [Page 79] Again, the aerial mammals, the bats, resemble those flying reptiles of the secondary epoch, the pterodactyles; not only to a certain extent in the breast-bone and mode of supporting the flying membrane, but also in the proportions of different parts of the spinal column and the hinder (pelvic) limbs. Also bivalve shell-fish (_i.e._ creatures of the mussel, cockle, and oyster class, which receive their name from the body being protected by a double shell, one valve of which is placed on each side) have their two shells united by one or two powerful muscles, which pass directly across from one shell to the other, and which are termed "adductor muscles" because by their contraction they bring together the valves and so close the shell. [Illustration: CYTHERIDEA TOROSA. [An ostracod (Crustacean), externally like a bivalve shell-fish (Lamellibranch).] Now there are certain animals which belong to the crab and lobster class (Crustacea)--a class constructed on an utterly different type from that on which the bivalve shell-fish are constructed--which present a very curious approximation to both the form and, in a certain respect, the structure of true bivalves. Allusion is here made to certain small Crustacea--certain phyllopods and ostracods--which have the hard outer coat of their thorax so modified as to look wonderfully like a bivalve shell, although its {80} nature and composition are quite different. But this is by no means all,--not only is there this external resemblance between the thoracic armour of the crustacean and the bivalve shell, but the two sides of the ostracod and phyllopod thorax are connected together also by an adductor muscle! [Illustration: A POLYZOON WITH BIRD'S-HEAD PROCESSES.]