RODENTIA-JACULUS. 
429 
JACULUS, Wagler. 
Jaculus, Wagler, Syst. Amph. 1830, 30. 
Meriones, Aud. and Bach. II, 1851, 251. 
Upper incisors grooved longitudinally on their anterior face. Molars the upper anterior one very small. Hand 
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 tbe 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.) Tbe 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 coecum, 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. 
