ON CERTAIN DEPOSITS OF SEMI-FOSSIL SHELLS 



IN HAMAKUA DISTRICT, HAWAII, 



WITH DESCRIPTIONS OF NEW SPECIES. 



By H. WETHERBEE HENSHAW. 



(Plate V.) 



TiiK island of Hawaii, the youngest as well as the largest of the Hawaiian 

 group, has reeeived comparatively little attention from conchologists. This 

 neglect no doubt is in no small degree due to the fact that the Achat inellidae, 

 which have chiefly excited the interest of students, are but sparingly repre- 

 sented on Hawaii, there being but three species, and these by no means the 

 most conspicuous of the group. The discovery, therefore, of deposits of 

 fossil, or rather of semi-fossil, shells in the Hamakua district, containing a 

 number of undescribed species, is not without interest. Before adverting to 

 the deposits, a brief description of the region is necessary to a proper under- 

 standing of their relation to the present fauna. 



The districts of Kohala and Hamakua comprise the northern end of the 

 island of Hawaii. This is by far the oldest part of the island, and probably 

 it had grown ancient before the fires that resulted in the upbuilding of the 

 huge mountain mass of Mauna Kea to the south, and the still more recent 

 Mauna Loa, were yet kindled. With little doubt then the northern end of 

 Hawaii was first to receive its fauna from the much older islands to the 

 north, and it in turn served as a nursery for the rest of the island as its lavas 

 were upheaved from the depths of ocean, cooled, and were slowly clothed 

 with vegetation.* 



^ The extraordinary development o( ihe Sued neii/ae \xpon the island of Hawaii suggests the possible 

 hypothesis that this island, the present metropolis of the group, was the first of the Hawaiian islands 

 to receive the .SuLcrnea stock, and that the other islands have been successively colonized from it 

 rather than the converse. Even when the present comparatively extensive deforestation of the more 

 northerly islands of the group is taken into account, the relative scarcity of the .Succineidne upon tbtm 

 is difficult to understand Dating back in origin to a vastly greater antiquity than Hawaii, the north- 

 ern islands would naturally be assumed, upon a priori grounds, to have received their Succinea stock 

 first ; hence they should be richer in species than the comparatively recent island ; the contrary is true. 



There is, howev-r, one important factor of the problem not to be overlooked. The Succineitiae 

 appear to have had the island of Hawaii pretty much to themselves from the very first, the compara- 

 tive scarcity of other land shells there, leaving them practically without competition. Thus favouied 

 by a comparatively free field, and with a general environment extremely favourable to their habits, 

 the Sticcineas, though perhaps with a long start upon the other islands, having later obtained a foot- 

 hold upon the big island may have attained their present rich development upon it in a comparatively 

 short time. 



As a factor in the development of the species of a group, lime would appear to be a less important 

 element than favourable environment and, above all. freedom from direct competition. 



The principal competitors of the Succineas for food are the genera Aviastra and I.eptachatina, 

 perhaps Carelia, and the Zonitidae. All of these, but especially Amastra, live chiefly upon 

 decaying vegetation and perhaps upon the fungi found theieon. The island of Hawaii is poor in 

 species of all these groups, Carelia being wholly absent, nor as a rule are any of these species 

 strongly represented by individuals. 



Upon Oahu the AcatinelLidae nrooer, the Aiitastras and the genus Auricula have attained a 

 development equalled upo'i no other island although both Maui and Molokai are greatly favourad by 

 the former. Upon Kauai tne genus Le/>tachatina has differentiated an extraordinary number of 

 species (many yet undescribed), and this genus and Carelia, the latter found upon no other island, 

 appear to have preemated the field. Thus several of the islands seem to have favoured one or two 

 groups which, having once gained the ascendency, have been able to hold it against all molluscan 

 competitors. 



