The comparative morphology of mammalian middle ear structures has been studied by examining the histological sections of the ears of 144 specimens of mammals which had been collected, with a few exceptions, in two expeditions to the Caribbean area. Fifty-four species of six orders of mammals were represented. Sixteen structural features or relationships were specifically studied and the bearing of taxonomic groupings on the variability was analysed.

A taxonometric analysis using these middle ear features indicated that Microdipodops may lie nearer to Dipodomys than to Perognathus.

A table presenting the detailed results has been deposited with the British Museum (Natural History).