Genus B. Carcinoscorpius with one species, C. rotundicauda (Latreille) (= L. rotundicauda, Latreille). It occupies a more westerly area than T. gigas or than T. tridentatus, having been recorded from India and Bengal, the Gulf of Siam, Penang, the Moluccas, and the Philippines.
With regard to the affinities of the group it is now almost universally accepted that they are Arachnids. The chief features in which they differ from other Arachnids are the presence of gills and the absence of Malpighian tubules, both being features associated with aquatic life. As long ago as 1829 Straus-Dürckheim emphasised the points of resemblance between the two groups, and although the view was during the middle of the last century by no means universally accepted, towards the end of that epoch the painstaking researches of Lankester and his pupils, who compared the King-crab and the Scorpion, segment with segment, organ with organ, tissue with tissue, almost cell with cell, established the connexion beyond doubt. Lankester would put the Trilobites in the same phylum, but in this we do not follow him. With regard to the brilliant but, to our mind, unconvincing speculations as to the connexion of some Limulus-like ancestor with the Vertebrates, we must refer the reader to the ingenious writings of Dr. Gaskell,[[222]] recently summarised in his volume on “The Origin of Vertebrates,” and to those of Dr. Patten in his article “On the Origin of Vertebrates from Arachnids.”[[223]]
Fossil Xiphosura.[[224]]
Fig. [159].—A., Hemiaspis limuloides, Woodw., Upper Silurian, Leintwardine, Shropshire. Natural size. (After Woodward.) B., Prestwichia (Euroöps) danae (Meek), Carboniferous, Illinois, × ⅔. (After Packard.)
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).
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.