August 16, 1895.] 



SCIENCE. 



181 



species of fish feed upon snails as a regular 

 diet. After feeding in one spot they repair 

 in schools to some particular spot, often 

 many miles distant from the feeding ground, 

 where they digest the animal and eject the 

 shell, either through the mouth or with the 

 fseces. In this manner litoral moUusks are 

 carried to the abyssal regions, and, I believe, 

 have been described as deep-sea forms. 

 The greater portion of the foregoing ac- 

 counts relate only to the dead shells which 

 have been ejected from the stomach of some 

 vertebrate animal, and the change cannot 

 be said to be a correct variation in habitat, 

 save in the few cases cited where the living- 

 animal was transported ; it is simplj' the 

 dispersal of a skeleton from one point to 

 another. 



Dispersal of the living animal by means of bi- 

 sects. — It would hardly be supposed that in- 

 sects could in any way be the means of dis- 

 tributing moUusks, and yet I have nu- 

 merous records of such distribution. Some 

 time ago a water beetle was captured, which 

 had a Sphcerium attached to one of its legs ; 

 another species was captured with an An- 

 cylus attached to its wingcase. I have 

 records of other instances where the larger 

 water beetles had specimens of Physa, Pla- 

 norbis, and even Limncea attached to different 

 portions of them. In this manner living 

 mollusks have been transported from one 

 pond to another, and so many of the fresh- 

 water species have been distributed by the 

 humble means of a water beetle. 



Distribution by means of storms, ocean cur- 

 rents, etc. — We may now consider a change 

 of habitat which affects the living animal 

 more closely than in the cases previously 

 cited. Many portions of Florida bordering 

 the rivers and creeks are continually falling 

 away and being carried into the Gulf, and 

 so to Cuba, Yucatan and other parts of the 

 West Indies and Mexico. These trans- 

 ported masses consist of three trunks, en- 

 twined vines, branches and roots of trees 



covered with earth and vegetation. Many 

 times, during storms, whole tracts of land 

 are washed away and portions of them of 

 considerable size are carried many miles by 

 the currents. Upon all of these natural 

 rafts, mollusks are found which are trans- 

 ported to habitats a great many miles from 

 those in which they first appeared. In this 

 manner Helices, Bulimi, Pupce, Limncea, 

 Physa, Planorbis, and a host of species too 

 numerous to mention, are carried from the 

 Southern States to Cuba, Mexico and Yuca- 

 tan. ISTo doubt many of the species of land 

 and fresh- water shells found in Cuba which 

 are also found in the Southern States, es- 

 pecially Florida, were carried there in this 

 manner, or were carried from Cuba to the 

 States. 



In the Western States fresh-water and 

 land shells are continually being carried 

 from one point to another. In the early 

 spring, when freshets and floods occur, the 

 young fry, as well as the adult animals, are 

 carried from the headwaters of the Missis- 

 sippi Eiver to various places along its banks 

 upon driftwood, tree trunks and an innumer- 

 able number of natu.ral rafts of this chai-ac- 

 ter. In this manner the species of Unio have 

 reached a comparatively wide distribution. 

 So also in our Great Lakes the fry of mol- 

 lusks are carried from Huron to Ontario. 

 Mountain streams, during freshets, are po- 

 tent vehicles of transportation in the spring, 

 and no doubt many mollusks living at high 

 altitudes are carried by this means from the 

 mountains to the valleys and plains below. 

 As an example of this I cite the following 

 case, furnished by a reliable correspondent : 

 About two years ago a colony of Vitrina lim- 

 p)ida was found in a Pennsylvania town just 

 after a severe flood ; the species had not 

 been before observed, although the collector 

 had searched the locality for several years 

 preceding. These shells had undoubtedly 

 been brought down the Alleghany Eiver on 

 rafts during the flood, and the little colony 



