V1U 



FACTS AND OBSERVATIONS. 



A certain number of these species are repeated in more than one country ; but the " repeats " 

 cannot be removed without more time than is at my command. These numbers, therefore, sometimes 

 represent appearances *, as denned. Incomparably the larger number of species appear, respec- 

 tively, only in one area ; nevertheless exceptions are common. Here are a few. 



Murchisonia bellicincta . 



Pleurotomaria umbilicata 



Murchisonia bicincta . ; 

 gracilis . . 



perangulata 



tricarinata . 



Platychisma helicites . 



Countries. 



. 10 



. 8 



. 6 



. 5 



. 5 



. 5 

 5 



Countries. 



Euomphalus alatus ...... 5 



funatus 5 



Holopella obsoleta 5 



Cyclonema bilix 4 



trimarginatus ... 4 



Murchisonia articulata . 4 



But going back to genera, Table B (below) shows that the eight following genera have the 

 widest and most complete range ; and it is great. The reason may be that the Gasteropoda are not 

 nice in their food, that they can exist on several kinds of sediments, and frequent the moderate 

 depth which allows of the freest travel. 



TABLE B. 



Most of these countries are exceedingly large ; so that for 171 species of Pleurotomaria to 

 exist only in thirty-four countries would seem to imply moderate range; but many of these regions 

 would well bear subdivision, when the apparent spreading would be greatly extended. The fossi- 

 liferous area (Silurian) of Canada alone is 60,000 to 80,000 square miles. 



Of the 51 genera of this order, 19 are confined each to one district, which is very credible, 

 seeing that 12 genera have only one or two species each. Among the other 32 genera there 

 are degrees of dispersion. Taking the whole class, there are 894 species and 1254 appearances, 

 or one quarter more appearances than species ; but if we look down the list of individual genera, 

 appearance and species are the same in 31 ; in other instances the former exceeds the latter by one 

 half, one third, or one fourth in Murchisonia by one third, in Rhaphistoma and Platychisma by 

 one half. Trilobites are liable to a smaller amount of dispersion than this. 



The specific appearances of the Gasteropoda are 658 in the Old World and 595 in the New 

 World, the former number being greatly increased by M. Barrande's discoveries in the Prague area. 



Twenty-two genera, and nine or ten species, are common to Europe and America, the species J 

 of course being greatly in the minority. As to the increment and decrement of species in time, every 

 large area must have its own ; but there is a common feature almost everywhere, such as we see 

 in the following Table, which exhibits the British Silurian Gasteropoda in their rise and decline. 



* By the word " appearance " is meant, in these pages, only the presence in any area of a particular species or 

 genus which may or may not exist elsewhere. One species may, and often does, make many " appearances ; " for example, 

 Orthis sericea is in many areas. '' Appearance " applies to horizons as well as to areas. 



t The United States of America (latitude 40-42) and Bohemia are both very strong in Acroculia ; there are 47 

 species in each. 



J They are : Clyclonema ventricosa, New York and Wales ; Euomphalus angulatus, Pennsylvania, Norway ; Mur- 

 chisonia angustata, Wisconsin, Wales ; M. bellicincta, New York &c., Silesia, Scotland ; M. tricarinata, New York &c., 

 Sardinia; Ophileta compacta, New York &c., North Scotland ; Raphistoma lenticularis, New York &c., Wales &c. ; 

 -R. qualteriata, Labrador, Russia, &c. ; Subulites elongatm, Canada, Esthonia j Trochus helicites, Nova Scotia, England. 



