ORDER VI RODENTIA 103 



posteriorly. The strong zygomatic arch is perforated by the infraorbital 

 foramen, which frequently forms so broad a channel that not only the nervus 

 facialis but also a portion of the masseter muscle passes through it. In the 

 Hystricoidea and Anomaluroidea the size of this opening is so great that it 

 exceeds the orbit. The jugal is always well developed, and the lachrymal is 

 considerably expanded. The anterior palatal foramina are of considerable 

 size. The lower jaw often has a prominent crest for the attachment of the 

 masseter muscle. The condyle is rounded oval, not transversely extended, 

 and is set high up on the jaw. 



A clavicular element is frequently present in the shoulder girdle. 

 The bones of the forearm are, as a rule, capable of free supination. A 

 centrale is often present in the carpus. Scaphoid and lunar are frequently 

 fused. The first of the five digits is often less developed than the rest. The 

 pointed terminal phalanges are clawed. 



In the pelvic girdle the ilium is remarkable for its slender, triangular 

 form, the pubes and ischia are large, and the two pubic bones are united 

 in the median line to form a long symphysis. The femur often bears a 

 third trochanter. The fibula and tibia are not infrequently fused at their 

 distal extremities, but all of the tarsals remain distinct. The digits are 

 often five in number — rarely three, and in the latter case a fusion of the 

 metatarsals is often observed. 



In contra- distinction to the primitive structure of the skull and 

 of the skeleton, the dentition, even of the oldest known rodents, is 



2.0.3.3. 

 very highly specialised. The dental formula varies between ' ' ' ' and 

 1.0.2. 1.0.3.2. 



' ' ' • The canines have entirely disappeared, and the cheek teeth are 



separated by a long diastema from the incisors, of which there is, as a 

 rule, only one in each jaw, and that is the original I^. Only in the suborder 

 Duplicidentata is there a small P behind the large P. The incisors, called 

 in this order gnawing teeth, on account of their function, continue to 

 grow as their crown is worn down through use. The cheek teeth are 

 never complete in number. The premolars may be entirely lacking and the 

 last of the molars may be very small. With the exception of the first P all 

 the cheek teeth have similar construction. They are short, brachyodont 

 and rooted in some forms, and high, hypsodont, prismatic and open below, 

 even in old age being without roots, in others. The former have bunodont 

 or lophodont crowns, the latter consist of prisms or cylinders with patches 

 of enamel on their crowns, at times — the remnant of former transverse 

 ridges — or of compressed lamellae. Originally the upper cheek teeth had 

 three or four cusps and the lower ones four, with a crested anterior and 

 posterior border. Later the neighbouring cusps united to form ridges. 

 Since the enamel covering is worn down on the ridges of the crowns through 

 use, the enamel is often lacking in these parts even in the young tooth. 

 The valleys in the tooth thus form the enamel inflections or fossettes, which 

 disappear partly or wholly with further wear. 



The change in dentition is restricted in the rodents, as a rule, to the P ; 

 the / are renewed only in Duplicidentata. In the rodents which have four 

 cheek teeth, the three hind ones are M and the front one a P, which has 

 often no milk predecessor. In most cases, however, a milk tooth, D, precedes 



