140 DE. J. MUEIE ON THE FORM AND STEUCTUBE OF THE MANATEE. 



in cross diameter. Besides suspensory ligament derived from the transverse process of 

 what may be considered a sacral vertebra, there is another, equally strong, passing 

 inwards from the pubo-ischial region to the vulva, behind which it meets its fellow 

 of the opposite side. This interpelvic bridge appears to be the homologue of a sub- 

 pubic ligament. 



4. Cranium and Dentition. 



So often has the skull of the Manatee been described and figured, that I restrict 

 myself to a short notice of its interior, and to a few remarks on the foetal cranium. 



The sections chosen to illustrate the cranial cavity are the internal basis as opened 

 horizontally or with calvarium removed, and a longitudinal vertical section to the left 

 of the middle line. The former (fig. 36, PL XXV.), of the female specimen, has the 

 dura mater attached on the left side, but the bones have been cleaned on the right ; 

 the latter (fig. 37, PI. XXVI.), of the younger male, is part and parcel of the sectional 

 view of the body with membranes and organs in situ. 



The interior basis cranii may be likened in figure to a broad stirrup, being very square 

 across the supraoccipital region, and arching regularly round from the temporal to the 

 frontal region. It is flattish below, as is the vault ; but the sides of the entire cavity 

 are steep and but slightly arched mesially. The length and breadth of the cranial 

 cavity are nearly equal ; but the height is rather less than either. In the female skull 

 (that depicted in fig. 36) the two horizontal diameters were about o\ inches, the vertical 

 close upon 2f inches. 



The skull's walls are very remarkable as regards inequality of thickness. The vertex, 

 to say the least, is as solid a piece of bone as can well be conceived, whilst the sides, 

 especially at the temporal region, are quite the reverse of this — namely, a thin plate of 

 bone. To specify, and with reference to our figure, the nearly vertical section of the 

 frontal bone in this female was above three quarters of an inch deep, the temporal and 

 part of the parietal plates little more thau a line, the occipital wall less than half an 

 inch at its middle, but thicker at the sides. The structure of the bone also varies. 

 The frontal is, to an extreme, dense and compact, as, indeed, is the osseous consistence 

 generally ; only a film of diploe is apparent on the lateral walls, and the occipital 

 segment has a distinct (though finely cancellated) interior, with a thin outer vitreous 

 table. 



The internal basis cranii examined, as in our figure, with the dura mater retained on 

 one moiety, presents a notable difference on the two sides. In that with the membrane 

 remaining (the left side, but right of fig. 36, as seen in the Plate) there are two subequal- 

 sized oval fossae, divided by a nearly transverse arched membranous ridge. 



These correspond respectively to the posterior and middle fossse of, say, the human 

 skull. The anterior fossa of Man, in the Manatee is nearly perpendicular, or forms the 

 front cranial wall, and hence is only partially visible on looking directly downwards into 

 the cranial cavity. Those fossae present in Manatus contain the anterior and posterior 



