RODENTIA JACULUS. 429 



JACULUS, Wagler. 



Jaculm, WAGLEtt, Syst. Amph. 1830, 30. 

 Meriones, ADD. and BA.CII. II, 1851, 251. 



Upper incisors grooved longitudinally on their anterior face. Molars Iri, the upper anterior one very small. Hand 



3 3 



with four fingers and a rudimentary thumb ; foot with five toes. Hind legs and tail very long, the latter thinly haired. 



It may be stated, among the generic characteristics of Jaculus, that the under surface of the 

 hind foot from the heel is perfectly naked ; smooth as far as the first tubercle, then granulated 

 or paved. There is a distinct web at the bases of the toes. The antitragus is much developed, 

 so as to constitute a thickened rounded valve capable of closing the meatus. 



Further details of structure will be found under the head of J. hudsonius. 



The principal characteristics of the skull have already been given under the head of the sub 

 family. The cranium is full and large ; the muzzle long, especially the nasal bones, which 

 project considerably beyond the incisors. There is also a sharp crest along the junction of the 

 intermaxillaries, separating the incisors at their bases. The infra-orbital nerve is provided 

 with a distinct foramen, which opens from a canal formed by a plate of bone, which, starting 

 as a ridge, running along the alveoli, widens anteriorly, and, bending over, cuts off this passage 

 way from the great ante-orbital foramen, which gives passage to the masseter muscle. The 

 incisive foramina are very large, situated half in the maxillary, half in the intermaxillary 

 bones, extending backwards as far as the second molars ; the septum dividing them much 

 swollen anteriorly. The palatine bone has two foramina, situated just behind the palato- 

 maxillary suture, both entirely in the palatine bone. The auditory bullae are quite small, and 

 have their longer axis nearly transverse to the longitudinal axis of the skull, (much more so 

 than in the rats.) The petrosal bone appears quite loosely implanted in the cranium. The 

 zygomatic arch is very broad anteriorly, and perforated by a large foramen, as described. Its 

 lower edge is on a level with the crowns of the molars, even below the palate ; it becomes very 

 thin in its horizontal portion. The malar bone is of great extent, reaching along the whole 

 zygoma from the lachrymal bone anteriorly almost to the auditory bullae. The zygomatic 

 process of the temporal bone is flattened and bent downwards so as to be nearly parallel to its 

 fellow on the opposite side. The malar bone is attached along its lower edge. The posterior 

 part of this process, forming the outer wall of the glenoid cavity, is distinctly notched. 



The anterior or first upper molar is very small, with a single root. The remaining three 

 decrease rapidly in size from before backwards. The crowns in the specimen examined are 

 broad and very much worn ; there is an exterior line of enamel, with three or four islands in 

 each. The two anterior lower molars are very large and nearly of equal size ; the third is 

 small, about the size of the last upper molar, and only half as long as those anterior to it. 

 The lower molars are similar in proportions to the upper ; the small anterior one wanting. 



The incisors are slender and much compressed ; colored dark orange. The upper ones have 

 the outer half of the anterior face rabbetted down below the level of the inner half, with the 

 corners rounded off ; viewed laterally, therefore, the groove is very distinctly visible. 



The tibia and fibula are united ; there is a distinct ccecum, measuring in J. labradorius an 

 inch and a half. In this species, also, the vertebral formula is: cervical, 7; dorsal and lumbar, 

 19 ; sacral, 4 ; caudal, 29. The clavicle is quite large. 



