xii FACTS AND OBSERVATIONS. 



We know 57 American genera ; of these 16 belong exclusively to that hemisphere ; while the 

 European genera are 118, 69 of these being never seen out of that quarter of the globe. Some 

 genera have many species, more abundantly than the Echinodermata, but much less so than the 

 Gasteropoda, and still less than the Cephalopoda. Phacops has 96 species, Illcenus 92, Asaphus 

 90, Lichas 79, Cheirurus 77, &c. Thirty-one genera have each only one species ; and they never 

 leave their native area, save in one instance (Polyeres, Bohemia, France) . 



Forty-one genera are only seen in one country or area respectively, although each may have 

 three, four, or six species, their associated genera meanwhile spreading far and wide. A curious 

 example of wide diffusion we have in the genus Cromus, which has been met with in Bohemia, the 

 Arctic seas of America, and in Belgium. 



The largest genera have been subject to the widest dispersion. Thus Dalmania is seen in 

 twenty-one separate regions, Lichas in twenty-four, Phacops in twenty-five, Cheirurus in twenty- 

 six, Illcenus in twenty-eight, and Calymene in twenty-nine. Isotelus with seven species occupies 

 fourteen countries*. 



It is to be feared that no general conclusions can be made from these facts until we know more 

 of the physiological conditions of these animals. Some rich genera are very local, such as Cyrto- 

 metopus, Bathyurus, Dikelocephalus, Dionide. The sixty-one species of Proetus are each typical 

 of one area, except Proetus concinnus, P. latifrons, P. Stokesii. Only three of the Primordial genus 

 Conocoryphe are in two countries : C. coronata, Spain and Bohemia ; C. depressa, Wales and Texas 

 (U. S. A) ; C. minuta, New York and Wisconsin. Twenty of the species belonging to this genus 

 are in Wisconsin, eleven in Wales, and the rest are scattered. The species of Bronteus are prin- 

 cipally massed within the Bohemian basin (forty-three out of fifty-six), three being typical of one 

 country only. 



The same species may inhabit many regions. Encrinurus punctatus is in eight, and is in two 

 quarters of the globe. Dysplanus centrotus is in six countries, Bumastus Barriensis in ten, while 

 Calymene Blumenbachii and C. senaria are each in seventeen. 



Instances of great migration from east to west, and from west to east are many, and about 

 equal in number ; but the particulars about the time-stages have not yet been fully ascertained. 

 Other directions are not uncommon. 



Calymene Blumenbachii is only seen in the Trenton limestone, in America ; but in Europe it 

 reappears in the Wenlock of Wales. Encrinurus segmentatus is in the Hudson- River group of the 

 Island of Anticosti, to revive in the Wenlock of Dudley &c. Many such examples are at hand. 

 This may be called recurrence by migration. 



Recurrence among Trilobites is rather weak, seven per cent, in species, computing from the 

 sum of all at present known ; that is, 115 recurrents out of 1677 species. The largest genera have 

 the most recurrent species. They are Acidaspis, Cheirurus, Lichas, Phacops, Proteus. The 

 majority of the smaller genera have either only one or none ; but some of even the most prolific have 

 none, such as Ogygia, Amphion, Megalaspis, Remopleurides. The same species in these four genera 



* The following are some examples of genera with few species, yet seen in several countries : 



Sp. Cos. 



Pharostoma 4 5 



Plesiocoma 2 3 



Polyerea 1 2 



Polytomurus 1 2 



Sp. Cos. 



Rhodope 3 3 



Triarthus , 5 6 



Sp. Cos. 



Actinopeltis 3 7 



Basilicus 4 7 



Dysplanus 3 6 I Bumastus 6 12 



