ib THE BRITISH MAMMALS : ORDERS, FAMILIES, ETC. 



the hinder pair are modified or have disappeared. The skull joins 

 the backbone by means of two occipital condyles. The teeth, when 

 present, are in sockets in the prernaxillae, maxillae, and mandible. 

 In the higher forms the dentition is heterodont, that is, the teeth 

 are of different kinds, and diphyodont, that is, in two sets, the first, 

 or milk teeth, preceding the permanent series. In one case, at 

 least, there are traces of a pre-milk set, and in a few there seems to 

 be a fourth set. The bones of the skull are firmly united, only the 

 lower jaw and the bones of the ear and tongue being movable. 

 The lower jaw works on the squamosal, the quadrate having become 

 one of the ear ossicles. The upper jaw-bones and palatal-bones 

 form a bony plate dividing off the cheek cavity from the nasal 

 passages! The muscular partition known as the diaphragm 

 separates the thoracic cavity from the abdomen. 



The class is divided into two sub-classes, one oviparous, the 

 other viviparous. In the first, Prototheria, the mammae are repre- 

 sented by a temporary pouch into which the ducts of the milk 

 glands deliver; in the second, Eutheria. which includes the 

 marsupials, in which the pouch is of a different character, there are 

 twelve orders, of which we have six represented in this country, 

 these being, Chiroptera, Insectivora, Carnivora, Rodentia, Ungulata, 

 and Cetacea. 



In the Chiroptera the limbs are specially adapted for flight. 

 The presternum is slightly keeled, the scapula triangular, the 

 clavicle long and curved, the humerus strong, the radius long and 

 curved, the ulna rudimentary ; the wrist has six bones, the scaphoid, 

 lunar and cuneiform being consolidated, the thumb is prominent 

 and clawed, and the four long fingers carry the patagium, or thin 

 extension of the skin generally known as the wing, which, as a rule, 

 begins on the shoulder, extends above the humerus and radius to 

 the base of the thumb, forming the antibrachial membrane ; thence 

 it extends between the fingers to the foot, and thence, as the inter- 

 femoral membrane, to the tail ; this lower section being supported 

 by a cartilaginous process, the calcaneum or calcar rising from the 

 inner side of the ankle joint, and bearing on its outer edge the post- 

 calcaneal lobe. The legs are short and weak, the fibula usually 

 rudimentary, and the knee bends inwards, like the elbow, owing to 

 the rotation of the leg outwards by the wing membrane. The bones 

 are slender and have large medullary, canals. The backbone is 

 short ; the cervicals are broad owing to the large size of the upper end 

 of the spinal cord, which in the lower portion of the back becomes a 

 mere thread. The ribs are flat ; the pelvis is weak. Some of the 

 bats are frugivorous, some insectivorous, the latter having the longer 

 tails owing to a more powerful steering apparatus being needed for 

 quicker turns in the capture of the prey. 



There are two sub-orders, Megachiroptera, of which there are no 

 representatives in this country, and Microchiroptera. In the Micro- 

 chiroptera the crowns of the molars have sharp cusps and transverse 

 grooves, the palate does not extend behind the last molar, the index 

 finger is without a claw. There are five families, of which only two, 

 Rhinolophidse and Vespertilionidae, are represented in our list. In 

 both the tail is contained within the interfemoral membrane, or 



