[Illustration: FIG. 74_a_.—_Belemnoteuthis antiquus._ Supposed to be a Belemnite, with soft parts preserved.—Jurassic.—After Mantell.] I have reserved no space to notice the geological history of the other and higher group of Cephalopods, including the true Cuttles and Squids. This is perhaps less to be regretted, as, from the absence of external shells, they are likely to be much less perfectly known as fossils. So far as known, they are vastly younger than the Nautiloids, for no examples whatever have been found in the Palæozoic. They appear abundantly in the Mesozoic, but are there represented principally by an extinct group of squids (Belemnites and their allies, Figs. 74, 74_a_), remarkable for the great and complicated development of their internal support, which has a chambered float as well as a solid sheath. This family becomes extinct at the close of the Mesozoic, though the cuttles as a whole perhaps culminate in the modern. [Illustration: FIG. 75.—Cambrian Trilobites. _a_, _Paradoxides_. _b_, _Dikellocephalus_. _c_, _Conocoryphe_ (head). _d_, _Agnostus_ (head and tail).] The remarkable group of the Trilobites had precedence in order of time of the Nautiloid shell-fishes. No animal structures can well be more dissimilar than those of the two great groups of aquatic animals which popular speech confounds under the name of “shell-fishes.” Take a whelk and a crab, for example, and compare their general forms, the structure of their shells, and their organs of motion, and it is scarcely possible to imagine any two animals more unlike; and when we examine their anatomy in detail this difference does not diminish. They have, it is true, corresponding parts, and these parts serve similar uses, but in plan of structure they are wholly different. Yet both animals may live in the same pool, and may subsist on nearly the same food. If we attempt to find some common type which both resemble, we may trace the structure of the crab back to those of some of the marine worms with which it has some affinity, and those of the whelk to such creatures as the _Lingula_, which are supposed to have a resemblance to the worms. But still the two types, that of the Mollusk and the Articulate, are distinct even from their first appearance in the egg, nor have either any close affinities with the Protozoa, the Hydroids, or the Corals. [Illustration: FIG. 76.—Transverse section of _Calymene_. A Silurian Trilobite.—After Wolcott. _a_, Dorsal shell. _b_, Visceral cavity. _c_, Legs. _d_, Epipodite—gill-cleaner or palp. _e_, Spiral gills.]