﻿MEASUREMENTS. 



11 



extent or exact dates of the northward and southward movements 

 coukl then be given further than that the known southern records of the 

 hoary bat (South Carolina, Greorgia, Bermuda Islands) were all during 

 autumn and winter, and that the silver-haired bat occurred in spring 

 and fall about the light-house on Mount Desert Rock, 30 miles olf the 

 coast of Maine, a treeless islet where bats were at other times unknown. 

 In August and September, 1890 and 1891, I had an opportunity to 

 watch the appearance and disappearance of three species of bats, Lasi- 

 onycteris noctivagans, Lashirtis borealis, and Lasiurus cinereuSj at High- 

 land Light, Cape Cod, Massachusetts. The animals, which were not to 

 be found during the early summer, suddenly became numerous shortly 

 after the middle of August and remained abundant for about a month, 

 when they as suddenly disappeared. The regularity with which this 

 phenomenon occurred on the two successive years over which my obser- 

 vations extended shows that the migration of bats is probably as defi- 

 nite as to dates and paths as that of birds. ^ 



MEASUREMENTS. 



For general purposes of identification, ten measurements are useful. 

 These are : Total length, tail vertebrje, tibia, foot, forearm, thumb, longest 

 finger, height of ear from meatus, width of ear, and height of tragus. 

 The lengths of the separate phalanges of the fingers are important in 

 special cases only. 



The tables which accompany the descriptions of the different forms 

 contain average measurements of specimens selected from as wide a 

 range of localities as possible. Whenever the full complement of meas- 

 urements is given, it is to be understood that all have been taken from 

 alcoholic specimens by the writer. When the total length, lengtii of 

 tail, and the three measurements of the ear are omitted, the measure- 

 ments have been taken from the dried skin. In a few cases the skin 

 measurements are supplemented by the collector's measurement of total 

 length and tail veitebnie. The use of specimens preserved in alcohol 

 introduces a source of error in two measurements — total length and 

 length of tail. According to the strength of the preservative fluid, both 

 body and tail are to a varying degree shrank or relaxed, so that consid- 

 erable discrepancies in the averages of specimens taken at different 

 localities by different collectors may result. In general, it is probable 

 that these two measurements as given in the tables are a trifle shorter 

 than they would have been if taken from fresh material. 



It is unfortunate that detailed measurements of individuals can not 

 be published, since averages are of use for comparison with averages 

 only, and it often happens that a single specimen must be identified. 

 Averages, moreover, give no indication of the normal range of indi- 

 vidual variation at a particular locality. 



'A detailed account of the migration of bats on Cape Cod was published in Science^ 

 N. S., V, No. 118, pp. 541-543, April 2, 1897. 



