142 DR. J. MUEIE 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 

 (aquaeductus vestibuli X). 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 foetus. 



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 (/o^), which continue 

 backwards and divide the temporal from both. The tympanic [Ty) and squamo-malar 



' Bijdragen, pi. iv. fig. 13 — an upper view of skull, but which I have supplemented by two other sketches of 

 the same specimen (PI. XXII.). 



