_Limulus_ is an example of a persistent type. It appears first in deposits of Triassic age, and is found again in the Jurassic, Cretaceous, and Oligocene. In the lithographic limestone of Solenhofen in Bavaria, which is of Upper Jurassic age, _Limulus_ is common and is represented by several species. One species is known from the Chalk of Lebanon, and another occurs in the Oligocene of Saxony. No other genus of the Xiphosura appears to be represented in the Mesozoic and Tertiary deposits, but in the Palaeozoic formations (principally in the Upper Silurian, the Old Red Sandstone, and the Coal Measures) several genera have been found, most of which differ from _Limulus_ in having some or all of the segments of the abdomen free; in this respect they resemble the Eurypterida, but differ from them in the number of segments. In _Hemiaspis_ (Fig. 159, A), from the Silurian, the segments of the abdomen are divisible into two groups (mesosoma and metasoma) in the same way that they are in Eurypterids; the first six segments have broad, short terga, the lateral margins of the sixth being divided into two lobes, probably indicating the presence of two fused segments; the last three segments are narrower and longer than the preceding, and at the end is a pointed tail-spine. In _Belinurus_ (Fig. 160) from the Carboniferous, the two regions of the abdomen are much less distinct; there are eight segments, the last three of which are fused together, and a long tail-spine. In _Neolimulus_, from the Silurian, there seems to be no division of the abdomen into two regions, and apparently all the segments were free. On the other hand, in _Prestwichia_ (Carboniferous), all the segments of the abdomen, of which there appear to be seven only, were fused together (Fig. 159, B). [Illustration: FIG. 160.—_Belinurus reginae_, Baily, Coal Measures, Queen’s Co., Ireland, × 1. (After Woodward). ] In the Palaeozoic genera the median or axial part of the dorsal surface is raised and distinctly limited on each side, so presenting a trilobed appearance similar to that of Trilobites. In _Neolimulus_, _Belinurus_, and _Prestwichia_, lateral eyes are present on the sides of the axial parts of the carapace, and near its front margin median eyes have been found in the two last-named genera. In nearly all the specimens of Palaeozoic Xiphosura[225] which have been found nothing is seen but the dorsal surface of the body; in only a very few cases have any traces of the appendages been seen,[226] but, so far as known, they appear to have the same general character as in _Limulus_. _Aglaspis_, found in the Upper Cambrian of Wisconsin, has been regarded as a Xiphosuran. If that view of its position is correct, then _Aglaspis_ will be the earliest representative of the group at present known. Other genera of Palaeozoic Xiphosura are _Bunodes_, _Bunodella_, and _Pseudoniscus_ in the Silurian; _Protolimulus_ in the Upper Devonian; and _Prolimulus_ in the Permian.