In the helamys the jugal is broader, and extends along a little more than half of the ring ; the rest is completed by the lachrymal, and even by the frontal bone. The lachrymal in this genus has no hook-like process : after having shown itself external to the orbital 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 ; 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 alm 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.
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 laehrymals 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 completelly separates the sphenoid from the maxillary. he 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 prehensilis, 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 lachrytnal 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 (111yopotamus, Commer son ; Mug 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. Inferiorly, 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 alm 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 meni branouo space on each side, at the base of the union between the palatines and the in ternal pterygoid. In the pacas (Calogenys, 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 pre-orbital ring, causes the latter to be much elongated transversely. And towards its in ner angle there is an excavation resembling a long furrow or semi canal, which is really the infra-orbital canal. The jugal is much higher than it is broad ; the palatine extends into the palate as far forward as the first molar n rs,4,;+ io alIntlet 11;f1.1..11 . IA., .0 . ....