VIU INTRODUCTION. 



broad behind, terminating in a broad cartilaginous process. In the 

 Megachiroptera the segments of the mesosternum are distinct, in 

 most species of Microchiroptera they are anchylosed together in 

 adult individuals. 



The ribs vary from 12 to 13 pairs ; in most species they are much 

 flattened and separated by very narrow intervals ; in the Rhinolo- 

 johidw they are exceedinglj^ flattened, and in some species occasion- 

 ally found partially anchylosed together. 



Of all parts of the osseous system the skull is most variable in 

 form. This variability evidently depends chiefly on the different 

 kinds of food used by different species, requiring special adaptation 

 in the manducatory apparatus, changes in the form of which leading 

 to very considerable modifications in the form of the skidl. We 

 would expect, therefore, to find the skull most variable in the Phyl- 

 lostomidce (for the species of that family probably vary more in the 

 nature of their food than all the other species of Chiroptera) ; and 

 this is certainly the case. In two species of this family, Ohceronyc- 

 teris mexicana and Centurio senex, we find the most extreme modi- 

 fications of the skull observable in Chiroptera, the length in pro- 

 portion to the breadth being as 30 to 11 in the former, and as 18 

 to 12 in the latter species (compare Plates XXVII. & XXVIII.). 

 In the truly insectivorous families, the Rhinolophidce and Vesper- 

 tilionidoe, the skuU is least variable ; and in some of the genera of 

 these families the difference between the species in this respect is 

 very slight. 



The elevation of the brain-case above the face-line varies very 

 considerably in the different species of Vespertilionidee, Emhallonu- 

 ridce, and Phyllostomidce especially. In some genera, as in Furia, 

 Amorphoclieilus, and Mormops, the crown of the head appears greatly 

 elevated, in consequence of the peculiar manner in which the facial 

 bones are bent upwards, so that the facial portion of the cranio- 

 facial axis is almost at right angles to the basicranial; in Minio- 

 pierus, Natalus, and Thyroptera the brain-case in much vaulted and 

 raised above the face-line, although the cranio-facial axis is almost 

 in the same plane throughout. 



Postorbital processes of the frontal are wanting in most of the 

 species of Microchiroptera ; they are either very short and obtuse or 

 not developed in Ehinolophidai and Phyllostomidce. In some species 

 of Vespertilionidte (Vesperugo paehypus, e. g.) there are small blunt 

 processes. In most of the species of Nycteridce the frontal bones are 

 greatly expanded laterally, forming triangular processes, the base of 



