RODENTIA. 



377 



ring, it occupies a considerable space in the 

 orbital cavity ; but the entrance of the lachrymal 

 canal is concealed by the sub-orbital arch. 

 The bones of the nose are singularly robust ; 

 the ascending ramus of the intermaxillary is, 

 on the contrary, very narrow, even at the 

 point where it' joins the frontal. The pre- 

 orbital ring is of large size, and the malar 

 apophysis of the maxillary arises close to the 

 intermaxillary suture. A few lines behind the 

 incisor teeth the palatine interposes itself, 

 under the shape of a round shield-like plate, 

 between the sphenoid and the maxillary. 



In the echimys the jugal is very long, and 

 tolerably broad j the lachrymal is small, and is 

 furnished with a little hook-like process ; the 

 maxillary presents, inferiorly, in front of the 

 molar teeth, a small fossa and a malar apo- 

 physis, the margin of which is broad and 

 flattened. The bony arch of the pre-orbital 

 ring is simple, and not doubled posteriorly by an 

 ascending apophysis of the jugal, as is the case 

 in the jerboa ; or by the latter and the lachry- 

 mal, as in the helamys and the viscache. The 

 palatine is deeply indented posteriorly, but it 

 ascends into the orbit, and likewise appears 

 in the pterygoid ala, between the sphenoid 

 and the maxillary. The external pterygoid 

 alee do not extend transversely beneath the 

 foramen ovale. 



The capromys very nearly resembles the 

 preceding genus in the construction of its 

 face, but in it the jugal bone is broader, and 

 almost rhomboidal in its shape. The sphe- 

 noid also is in contact with the maxillary, 

 above the point of union between this bone 

 and the pterygoid portion of the palatine. 



Fig. 257. 



Skull of the Porcupine (Hystrix cristata). 



In the common porcupine (Hystrix cristata, 

 Lin.), the bones of the nose are very large and 

 broad, the suture connecting them with the 

 frontal mounts much higher up than the inter- 

 maxillary sutures. The intermaxillary bones 

 have their ascending ramus much less narrow 

 than in the preceding genera. The maxillary is 

 hence a pre-orbital ring, which is much broader 

 than it is high ; and the inferior horizontal por- 

 tion of its circumference is much more slen- 

 der than the vertical posterior portion. The 

 lachrymals consist of a small portion situated 

 external to the orbit, which furnishes a little 

 hook-like process, and of another portion 

 situated within the orbit, which is also of small 



size. The jugal is of moderate dimensions* 

 and broader in front than it is posteriorly ; the 

 palatine is deeply indented, and only sends off 

 a little tongue-like pointed process to pene- 

 trate the orbit ; but it completely separates 

 the sphenoid from the maxillary. The internal 

 pterygoid processes end in hook-like ter- 

 minations, the extremities of which are united 

 to the tympanum ; the external ones only re- 

 present a transverse bar, into the composition 

 of which the palatine slightly enters. 



In the cuendu (Hystrix prekensi/is, Lin.), 

 the bones of the nose are short and flattened 

 at their anterior portion ; they are likewise 

 remarkably broad and mount very high up. 

 The pre-orbital ring is higher than it is broad. 

 The internal pterygoid process extends as far 

 as the tympanum. In the ursons (Hystrix 

 dorsata, Lin.), the pre-orbital ring is larger than 

 in either of the preceding genera, and its two 

 arches are of equal strength ; the bones of the 

 nose are short, flat, and one third narrower 

 than those of the cuendu. In both genera, 

 the lachrymal is united, at an early age, both 

 with the maxillary and the frontal. The in- 

 termaxillary suture is straight and almost 

 vertical. In the coni (Myopotamus, Commer- 

 son; Mus coipus, Molin.), the bones of the 

 nose are broad and much elongated ; they do 

 not mount higher than the intermaxillaries. 

 The suture between the latter bones forms a 

 very rounded arch, which is concaved pos- 

 teriorly. The maxillary has the inferior edge 

 of its malar apophysis very much flattened. 

 The pre-orbital ring is large. The palatine is 

 in contact with the maxillary below, but above 

 the sphenoid joins that bone likewise, as in 

 Orycterus and other genera. 



In the agouti (Chloromys, Fred. Cuv. ; 

 Dasyprocta^ Ilig.), the lachrymal, which is 

 larger than in the allied genera, contributes to 

 surround the sub-orbital foramen superiorly, so 

 that the ring formed around this hole by the 

 maxillary is not complete. The lachrymal 

 comes down very nearly as far as the jugal 

 bone, but does not touch it. The jugal itself 

 is very small. The pre-orbital ring is broader 

 than it is high ; and there is, moreover, in- 

 ternal to it, situated upon the cheek just above 

 the commencement of the malar apophysis, a 

 long oval sinus, into which, both before and 

 behind, a rounded canal opens. Inferiorlv, 

 the palate bone advances in a wedge-like 

 manner as far as opposite to the first molar 

 tooth ; it penetrates into the orbit by a thin 

 slip, which separates the sphenoid from the 

 maxillary. The internal pterygoid alee are pro- 

 longed as far as the tympanum by a broad 

 hook-like process ; the"external form simple 

 plates, to the construction of which the pala- 

 tines partially contribute. There is a mem- 

 branoufi space on each side, at the base of 

 the union between the palatines and the in- 

 ternal pterygoid. In the pacas (Ccelosenys 

 Fred. Cuv. ; Cavia Paca, Lin.), the maxillary 

 portion of the zygomatic arch conceals be- 

 neath it an enormous sinus, which is less deep 

 in very young subjects than in adult animals. 

 This swelling, which fills up a portion of the 



