136 Bulletin American Museum of Natural History. [Vol. XX, 



the vertical forward; it cannot be pressed backward to close 

 the meatus without breaking the ligamentous hinge, but can 

 readily be flexed forward through a wide angle. Its func- 

 tion therefore seems to be to prevent the closing or partial 

 closing of the meatus by the soft parts of the ear. The ex- 

 ternal ear is not large, in this group, for the size of the animal, 

 as is the case in Chinchilla, where it also exists, so that this 

 appendage is not necessarily correlated with a large external 

 ear, nor even with greatly developed audital bullae. This 

 bone is of very dense structure, and is evidently developed 

 from an independent center of ossification. 



On examination of alcoholic specimens of Liomys bulleri 

 (Thomas), Heteromys anomalus (Thompson), and Heteromys 

 jesupi Allen, I find this appendage to be well developed in 

 each of these species, as it doubtless is in all the members of 

 the Heteromys (including Liomys} group. But I could find 

 no trace of it in the only species of Perognathus of which al- 

 coholics are available for examination. In the larger species 

 of Perognathus, as in the hispidus and penicillatus groups, 

 there seems to be a functional equivalent in the building up 

 of the anterior border of the bony meatus into a slightly 

 projecting lip. I have found it also absent in Zapus and 

 Proechimys, where it seemed likely to occur; but in the case 

 of the latter the base of the external ear forms a firm cartilagin- 

 ous tube. A glance at the skull of a Dipodomys or a Pero- 

 dipus is sufficient to show that no equivalent modification 

 need be looked for in these groups, owing to the posterior 

 position and backward opening of the meatus. Anatomical 

 examination of these forms has confirmed this assumption. 



This structure does not appear to be mentioned in general 

 works on mammalian anatomy, but there are incidental 

 references to it in other connections just, how many is 

 difficult to trace. I have thus far found only the following: 



In 1890 the late Dr. G. E. Manigault, of Charleston, South 

 Carolina, reported (Proc. Elliott Soc. Nat. Hist., May, 1890, 

 pp. 237-239) the discovery of a "crescent-shaped flat bone 

 occupying the anterior half of the outer edge of the external 

 meatus of the ear," in Chinchilla lanigera, and gave figures 



