461 



shed : the crown presents an irregular oval shape in transverse section. The fourth molar 

 tooth, when fully formed, resembles a slightly bent cylinder, with a nearly smooth outer 

 surface ; the crown is flat or slightly depressed at the centre. The opposite extremity of the 

 tooth is excavated by a regular conical cavity, lodging the remains of the pulp. With age, 

 however, the fang contracts, takes on an irregularly fluted and tuberculated surface, and is 

 at last closed at its extremity. The matrix of the last molar tooth expands as the crown is 

 forming, and manifests a tendency to divide into two fangs ; but, having acquired the size and 

 form shown in the specimen, the pulp is maintained hi a wide basal pulp-cavity to supply the 

 waste of the crown according to that pattern. 



Presented by Prof. Owen, F.R.S. 



2549. The disarticulate bones of the head of a Malayan Dugong (Halicore indicm). 



They are indicated by numbers on coloured labels according to the TABLE OF SYNONYMS. 

 The basioccipital is a triradiate bone, the two short rays diverging posteriorly to join the ex- 

 occipitals, and forming the lower end of each condyle. The exoccipitals almost meet above, 

 and complete the foramen magnum : they are grooved, not perforated anterior to the con- 

 dyle, and have a short, rough, subcompressed paroccipital process, which is perforated. The 

 superoccipital is firmly anchylosed to the parietals, which have equally coalesced into a single 

 subquadrate massive bone, with the sides bent down at nearly a right angle with the almost 

 flat upper part, which is perforated by a ' foramen parietale ' at the back part, near the con- 

 fluence with the occipital. The basisphenoid has coalesced with the orbitosphenoids as 

 well as with the alisphenoids : it has no sella turcica. The alisphenoids are grooved, not 

 perforated, by the trigeminal nerve. The pterygoids are anchylosed to the base of the ali- 

 sphenoids and the posterior ends of the palatines, which are wedged into the interspace between 

 the pterygoid and ectopterygoid processes. The orbitosphenoids are perforated by widely 

 separated optic foramina : they are anchylosed with the presphenoid, and these with the 

 slightly-developed ethmoid. The cribriform plates are not half the size of those in the Manatee, 

 and are lodged in deep fossae surrounded by rugous portions of the ethmoid, homologous with 

 the cells in laud mammals. There is no crista galli. The frontals are not confluent ; their 

 orbital processes extend far forwards and outwards from the anterior angles: the median 

 angles of the nasal border are slightly produced, but there is no trace of a suture marking 

 out the proper nasals. The frontals are excavated almost to the posterior margin by the nasal 

 cavity. The cranial plate of the frontal forms a small concave surface, not exceeding the 

 depth and thickness of the posterior part of the bone to which it is confined : the orbital pro- 

 cesses extend far forwards and outwards beyond the nasal part, which is excavated as far back 

 as the coronal suture. 



The maxillary is deflected anteriorly ; its nasal and malar processes do not meet and cir- 

 cumscribe the great antorbital foramen, but this is closed by the upper end of the malar bone. 

 The premaxillary is remarkable for its very large and long deflected alveolar portion, and for 

 its slender elongated nasal portion : the base of the alveolus is open externally, indicating the 

 female sex. The squamosal forms no part of the inner surface of the cerebral cavity, but is 

 deeply and smoothly excavated for the lodgement of the dense petromastoid, with which the 

 tympanic bone is anchylosed. The mastoid part forms a small rugged surface, wedged he- 



