In the Prionidæ there are 6 peculiar genera, the largest being _Holonotus_ with 3 species; two others, _Derotrachus_ and _Mallaspis_, are characteristic; 3 more are common to South America, and 1 to Cuba. The Cerambycidæ are much more numerous, and there are 24 peculiar genera, the most important being _Sphenothecus_, _Entomosterna_, and _Cyphosterna_; while _Crioprosopus_ and _Metaleptus_ are characteristic of the sub-region, although extending into South America; about 12 Neotropical genera extend to Mexico or Guatemala, while 12 more stop short, as far as yet known, at Nicaragua. Lamiidæ have a very similar distribution; 13 genera are peculiar, the most important being _Monilema_, _Hamatoderus_, and _Carneades_, while _Phæa_ and _Lagochirus_ are characteristic. About sixteen typical Neotropical genera extend to Mexico, and 15 more only reach Nicaragua, among which are such important genera as _Anisopus_, _Lepturgus_, and _Callia_. The land-shells are not sufficiently known to furnish any corresponding results. They are however mostly of South American genera, and have comparatively little affinity for those of the Antilles. _Relations of the Mexican sub-region to the North and South American Continents._--The sudden appearance of numerous South American forms of Edentata in temperate North America, in Post-Tertiary times, as narrated in Chapter VII., together with such facts as the occurrence of a considerable number of identical species of sea fish on the two sides of the Central American isthmus, render it almost certain that the union of North and South America is comparatively a recent occurrance, and that during the Miocene and Pliocene periods, they were separated by a wide arm of the sea. The low country of Nicaragua was probably the part submerged, leaving the highlands of Mexico and Guatemala still united with the North {58}American continent, and forming part of the Tertiary "Nearctic region." This is clearly indicated both by the many Nearctic forms which do not pass south of Nicaragua, of which the turkeys (_Meleagris_) are a striking example, and by the comparative poverty of this area in typical Neotropical groups. During the Miocene period there was not that marked diversity of climate between North and South America that now prevails; for when a luxuriant vegetation covered what are now the shores of the Arctic Ocean, the country south of the great lakes must have been almost or quite tropical. At an early Tertiary period, the zoological differences of the Nearctic and Neotropical regions were probably more radical than they are now, South America being a huge island, or group of islands--a kind of Australia of the New World, chiefly inhabited by the imperfectly organized Edentata; while North America abounded in Ungulata and Carnivora, and perhaps formed a part of the great Old World continent. There were also one or more very ancient unions (in Eocene or Miocene times) of the two continents,