179 



Length of the row of the molares on each side 1 1 



Breadth of the lower jaw from the outer edge of one 



ramus at its broadest part to the opposite 3 3 



From the middle of the condyle of the lower jaw to the 



base of the incisors 3 1 



" Placing the skull before us, and surveying its upper aspect, we 

 observe that the nasal bones are narrow and elongated, being broadest 

 at their nasal extremity and narrowest at their frontal, as in Cupromys 

 and the Water Rat, but not in so great a degree. In the Cupybara 

 the contrary obtains : in the Beaver the nasal bones are broadest in 

 the middle. Their length in the Coypus is I4 inch, their united 

 breadth at the frontal union 5 lines. 



" By the side of the nasal bones runs up the ascending ramus of 

 the intermaxillary bone, which at its union to the frontal expands 

 considerably, and terminates on an exact level with the nasal. In 

 the Capybara the ramus is very narrow, and does not ascend quite so 

 high as the nasal : in two skulls of the Beaver now Ijefore me, I find 

 it ascend 1 line -higher than the nasal in the one, and 2 lines lower 

 than the nasal in the other. In Capromys it ascends somewhat 

 higher. 



"The frontal bones, having a longitudinal suture between them, 

 form an oblong square, occupying a considerable space, their length 

 being 1 inch 5 lines, and their united breadth 1 inch 3 lines. They 

 form above the orbits a bold but level ridge : in the Capybara this 

 ridge is arched, rounding the orbit above ; in the Beaver the ridge is 

 but little prominent ; and in the Wafer Rat there is none. It may 

 be added that in the Beaver the frontal bone (for here we may 

 speak of it as single) approaches a triangle in its outline, the anterior 

 portion of the parietal bone on each side advancing upon it. 



" In the Coypus the parietal bones are small, and are depressed on 

 each side posteriorly to form a deep temporal/ossa, bounded by a ridge 

 (the index of the origin of the temporal muscle), which ridge, with 

 the coronal suture for a base line, forms a triangle ending in a slight 

 short sagittal crest. The parietal bones are nearly consolidated to- 

 gether, and doubtless become ultimately completely so ; it is only 

 for a short space from the coronal suture that in the present skull 

 any trace of a sagittal suture is visible. In the Capybara the union 

 is complete ; but in the Beaver the sagittal suture continues unobli- 

 terated, and the parietal bones moreover are separated posteriorly by 

 a large interparietal or os triquetrum. In the Water Rat there is an 

 oblong post-parietal bone. 



"The occipital bone is narrower than in the Beaver, and more 

 nearly resembles that of the Cajjybara ; it rises, however, immediately 

 behind the lambdoid suture into a high strong transverse crest, which 

 sweeps down on each side, and is continued in two strong processes, 

 the outer and shorter of which passes just behind the auditory yb- 

 ramen, while the interior process has its base Ijetween the former and 

 the condyle, abuts upon the posterior part of the tympanic bulla, and 

 passing obliquely outwards and downwards ends in a broad lunar- 

 shaped termination : its length is 1 inch 2 lines. The foramen 

 magnum is nearly circular : in the Beaver it is compressed horizon- 



