XXV1 INTRODUCTION. 
The Table given of the distribution of the Terrestrial Mollusca shows, further, a 
much larger number of species in North Guatemala than in what I have termed 
Central Guatemala; this difference is not due to the extent of the district or 
its lower elevation, or to the greater number of collectors, but chiefly to the 
geological formation of the soil. I limited “Central Guatemala” to the Province of 
Baja Vera Paz, occupied chiefly by metamorphic rocks, including also the drainage of 
the upper part of the River Usumacinta (Rio Salinas) to the north, and that of the 
Rio Grande to the east; the Province of Alta Vera Paz, which is mostly limestone, 
is here included in North Guatemala. Now in all parts of the world a limestone- 
formation is much richer in land-shells than a plutonic or volcanic one, not only with 
respect to the number of species, but still more as regards the number of individuals. 
In countries which have not yet been thoroughly explored the number of the 
individuals influences also the number of known species, as those which are most 
abundant are, of course, the first to be noticed by the passing traveller. The greater 
richness of this formation in land-shells is, however, not exclusively due to the chemical 
composition of the soil, but in large degree also to its physical qualities: limestone 
offers generally a very diversified aspect, with rocks and holes, and projecting edges 
and re-entering corners, more or less open to the sun’s rays and yet offering 
everywhere shady places for retirement during the heat of the day. 
Concerning Guatemala, Mr. Champion has sent me the following note:—‘‘If you 
travel from the limestone regions of Vera Paz (Coban, Senahu, Cubilguitz, &c.) to the 
volcanic districts of the central tableland of Guatemala, you find a vast difference in 
the land-shells—many species and individuals in the former, scarcely any in the latter. 
The absence of the limestone, in my opinion, explains the absence of many forms, 
quite apart from the geographical situation.” Dr. Stoll, in his work on Guatemala 
(1886), pp. 196-199, says, concerning the land-shells of the Pacific slope as 
compared with those of Alta Vera Paz:—“ The district of Cholhuitz, 2000-4000 feet 
(640-1280 m.), on the advanced slopes of the Volcan de Santa Maria, which has 
deeply-cut ravines (barrancas) and virgin forest, with copious rainfall and a warm 
climate, offers a very rich Molluscan fauna, which, however, is rapidly decreasing by 
the annual cutting and burning of the trees; this fauna shows a very remarkable 
conformity with that of Alta Vera Paz, from which it is separated by large, barren, 
elevated tracts of land. In both districts live Helix ghiesbreghti, H. eximia, 
and H. trigonostoma, Otostomus delattre: (on the Pacific slope represented by the 
