142 DR. J. MURIE ON THE FORM AND STRUCTURE OF THE MANATEE. 
of the great foramen magnum. But the two more interesting phases of the interior 
osseous cranial construction are an immense fissure (a continuous foramen lacerum me- 
dium and posterius) and as remarkable a development of the periotic (Per). The great 
fissure spoken of forms a considerable segment of a circle, broad and irregularly con- 
toured in front, and narrowing as it sweeps inwards and then round the periotic. It is 
bounded laterally, forwards, and internally, respectively, by two divisions of the periotic 
presently to be mentioned, a tip of the wedge squamo-parietal, the posterior border of 
the alisphenoid, and by the basioccipital. Its narrow posterior horn, or what corresponds 
to the jugular portion, dips between the posterior border of the periotic and exoccipital, 
and communicates with the great inferior basal petrotympanic cavity. The massive and 
dense periotic within the skull is bicuspid, and occupies nearly half the interior. The 
anterior smaller division partially constitutes the lateral cranial wall, and abuts upon 
the squamo-parietal wedge behind the alisphenoid. The posterior larger division (= pars 
petrosa) juts across the cranial basis, as a thick nodular mass, behind the above-men- 
tioned foramen lacerum medium. Its upper moiety is swollen, a prominent node marking 
the semicircular canals (sc*), on the posterior surface of which is a vertical fissure 
(aqueeductus vestibuli?). The lower moiety is separated from the upper by a transverse 
sulcus, superior petrosal groove, near the anterior end of which is the meatus auditorius 
internus (7), and above and forwards by two foramina (=hiatus Fallopius and lamina 
cribrosa ?). ; 
The great cranial fissure is ordinarily closed above by the dura mater, as has been 
shown; and beneath this is a large sac, connected with the Eustachian tube, and com- 
municating with the tympano-periotic fossa. The lower wall-membrane of this sac 
reaches from the alisphenoid to the exoccipital and stylo-hyal cartilage, and crosswise 
from the basiocciput to the tympanic. 
The youngest Manatus skeleton which I have had access to is that in the Amsterdam 
Zoological Gardens, and said by Vrolik, in his memoir, to be that of a feetus. 
Each half of the inferior maxillary bone apparently has had three centres of ossifica- 
tion, at least is suturally divided into three areas (1, 2, 3, fig. 16)—namely, symphysial, 
angular, and ascending ramal divisions. The sutural lines of demarcation spring 
triradially from the proximal end of the body of the bone, and are pretty regular in 
their course, that across the ramus being the longest. The frontal bone (fr) is 
bilateral, as Vrolik has shown'; and a large fontanelle mesially divides the parietals 
backwards to the supraoccipital. The coronal suture runs in nearly a straight line 
across the vertex. A parieto-squamal suture is well defined. ‘The supraoccipital (So) 
is a single transversely oval-figured bony area, quite separated from the exoccipital by 
interfibrous material, and laterally bounded by broadish fontanelles (fo*), which continue 
backwards and divide the temporal from both. The tympanic (7) and squamo-malar 
) Bijdragen, pl. iv. fig. 13—an upper view of skull, but which I haye supplemented by two other sketches of 
the same specimen (Pl, XXII.), 
