6 BULLETIN 43, UNITED STATES NATIONAL MUSEUM. 



the small thumb to a point beyoml the first phalanx of the thumb- 

 when It extencLs down to the foot beyond an oblique muscle line which 

 extends upward and outward from the lower part of the leg- when the 

 space between the second and third digits is ample, and that between 

 the thumb and second digit is provided with a well-defined hem of 

 membrane. 



Skin folds are ofteji disposed along the lines represented by the 

 palmar lascia, at the proximal end of the fifth digit.* The flexor ten 

 dons at the radio-digital angle are often covered with similar disposi- 

 tions of the skin. 



The membranes are supported not only by the parts of the skeletal 

 frame-work, as these parts are usually defined, but by a number of 

 special adaptati(n.s. An accessory cartilage at the somad margin of 

 the teiinmal fifth digit has been already named (Yespertilionid.^, ex- 

 cept Plecoti).t The interfemoral membrane is supported at the free 

 margin hya special cartilage (calcar) from the tarsus in all bats except- 

 ing the 1 teropidc^, Ehinolophidas and the stenodermata. The calcar 

 may have a process from its under margin, as in JSroctuHnia noctida. 

 1 he terimnal j,.i„t of the tail may be spatulate, as in mjcteris. Termi- 

 na cartdages of the third and fourth digits are present except in Ptero- 

 pidse, Ehinolophidi^, and EmballonuridiP. They are of varying 

 shapes, the whole arrangement having for its object the support of the 

 Ire^ n.argm ot the wing membrane. These cartilages, as a rule, are 

 deflected outward, though they may remain axial, as in Phyllostomid.e 

 and Plecoti. 



All things remaining the same, the degree of strain may be measured 

 by the extent and variety of these special supports, and mav be said 

 to be in the line of specialization for aerial movements. Hence; in forms 

 m which they are absent the membranes are broad and may be said to 

 exhibit more of a paracliute arrangement than in other types in whicli 

 they are present, and the motion of the wings to be like that of a slow 

 tanning rather than a rapid, varied flight. 



Strain on the membranes is also shown in the angles formed between 

 the portions of the wing fartliest away from the body, namely, tlie 

 region of the second and third digits. These are pulled away from the 

 fourth and fiftli digits, which remain nearly passive, by the traction of 

 the muscles which extend tliese bones (extensores carpi radiales longior 

 et brevior), and the whole membrane becomes tense. The contrast be- 

 tween the shapes of the wing in this regard is considerable when such 

 forms as Artibens, ^-yrfinomus, and Atalapha are compared. 



When the wing of a bat is held up between the eye of the observer 



^Vespertilionid.-P (excepting Plecoti), and Molossi. 



t In the prodrome of this introductory essay (Proc. U. S. Nat. Mns. xvi. 1893) this 

 Iha e «i/' ^'''" T '■;' f^'''''''^^^^^ ^^^«l"««i- -l^ile said to he absent in A.trozous. 

 I hav e since corrected hoth of these statements. My first announcement respecting 

 the accessory cartUage was made in Proc. Acad. Nat. Sci. Phila., 1889, 330 ,^e)^ 



