16 
YORKSHIRE NATURALISTS' UNION. 
related, and at one time had a continuous and cunipaet range, and 
must have emanated irom the same ancestral source. 
The Helirtdtv are divided by Dr. Pilsbry into four i;hiel' groups : 
Belogona niphrmadriuit, BeUtgond eiuidenid, Epiithallogona, and 
Pvfftogona ; while a more generalized group, the Haplixjona, which 
stand near the common ancestor of tlie llellcida- and certain other 
famihes, will also be considered. 
The Belogona siphonadknia exemplify the highest development 
attained by the HAk 'tdcc, and are dominant throughout the world, 
possessing a most remarkable power of accommodation to almofst any 
conditions, evidenced by an almost uniciue cai)acity for colonization, 
and an extraordinary faculty of supplanting and driving oft' the 
indigenous or native species of every other region, an endowment 
they share with the human race of their native country. 
The highest development of this .section is in North Central 
Europe, the most advanced region in the world, and the theatre of 
the highest development in plants and animals, from whence they 
are spieading in every direction that the physical barriers will allow ; 
the more ancient and primitive species of the group have spread 
beyond the limits of Euroi)e, having crossed the Asiatic continent, 
and now overlap and mingle with the rear of the less dominant 
Euadeniate group which preceded them. 
The Belogona euadenia are an earlier form of the last section, 
diff'ering in the less perfect dart a])])aratus and sacculate glands, 
and at the ])resent day have their metropolis in China and Eastern 
Asia, though, undoubtedly, originating in the European region prior 
to the Siphonadeniate species, by whom they aie being gradually 
expelled therefrom, although many species are still found along the 
migratory track, and a single species of Ealotd, one of the latest 
evolved of its group, still lingers in Europe, a solitary representative 
there of this formerly dominant race, whose former presence in Europe 
is also indicated by the intimate relationship existing between 
tertiary and living Euroi)ean species and certain Chinese forms. 
Though thus retreating from Europe, its earlier evolved members, 
typified by HeUcos^tyUt, with its primitive dart apparatus, have 
penetrated to New Guinea and the Solomon Islands, and overlap and 
compete with their more archaic piedecessors, the EpiphaUofjomi. 
Although now a waning grou]) in Palsearctica, it is still the most 
dominant llelicidian group in the New World, where it is found 
along the Pacific coast of that continent, penetrating .to South 
America and the Greater Antilles. 
The Epiphallogona are a still more ancient group, and though 
quite destitute of the love-darts, etc., so characteristic of the two 
preceding grou})s, blends im))erce})tibly with them. It was the earliest 
group to ac(iuire the E|»iphallus, an organ in which the male element 
is collected and (-(jusoiidated and transferred bodily when ])airing. 
The Epiphdllogomi, by their distribution and structure, ])lainly show 
themselves to be the predecessors of the Euadenia, and evidently arose 
