90 MB. /W. H. PLOWBB ON THE OSTEOLOGY OP 



nication (see Plates XXV. & XXVI.), the elements of the occipital bone have completely 

 coalesced with each other and with the basisphenoid, and partially with the parietals. 

 The foramen magnum is subcircular, the greatest vertical and transverse diameters being 

 exactly equal ; but it is rather broader above than below. Its plane is nearly vertical 

 when the skull is held horizontally. The condyles are large and prominent ; they do not 

 meet below by a space of -l". In the middle line on the supraoccipital, just above the 

 margin of the foramen magnum, is a deep triangular depression, continuous with a 

 broad and shallow median groove which ascends nearly to the vertex, and with lateral 

 grooves which jiass outwards above the upper edge of the condyles to the concave surface 

 of the exoccipitals. In the lower part of the median groove the surface of the bone 

 is very rough, being channelled out for a plexus of blood-vessels ; and there are several 

 rounded perforations, one of them as much as '1" in diameter, by which these vessels 

 would apparently communicate with the interior of the cranial cavity. Corresponding to 

 this groove, on the inner side, is a median bony ridge, but there is no transverse tentorial 

 ossification. The lateral boundaries of the supraoccipital s are raised into strong narrow 

 ridges, on the summit of which the occipito-parietal suture is situated. These are nearly 

 parallel until they come opposite to the posterior angle of the maxillaries ; then they 

 rapidly converge, enclosing a triangle with a truncated apex which projects forward 

 into the high postnarial eminence of the fi-ontals. 



The temporal fossa, as noticed by d'Orbigny, is very much larger in proportion to the 

 size of the. cranium than in any other Dolphin, except Platanista, not only occupying a 

 larger space on the lateral surface of the skull, but being prolonged forward at the 

 expense of the orbit. Its form is that of a long oval, with the small end turned forwards. 

 Its posterior nearly semicircular boundary is formed by the ridge, before spoken of, at the 

 junction of the occipital with the squamosal and parietal. The superior border, continued 

 forwards from the latter, is a nearly straight, sharp, thin crest, projecting outwards and 

 upwards, 3" long, and averaging more than half an inch in height, formed by the 

 maxillary uniting with the edge of the frontal, and posteriorly with the parietal. The 

 inferior border is formed by a long and strong zygomatic process of the squamosal, 

 approaching, but not equalling, that of Platanista in size, and a triangular pointed 

 postorbital process of the frontal, -7" in length, and directed backwards and downwards, 

 but which does not meet the process of the squamosal, by a space equal to its own 

 length. In Platanista there is no space or postorbital process, the anterior end of the 

 prodigiously developed zygomatic process of the squamosal reaching so far forward as 

 even to be lodged in a hollow in that part of the orbital plate of the frontal from 

 which such a process is usually developed. 



• The bones which enter into the formation of the temporal fossa resemble in their 

 number and arrangement those of the true Dolphins rather than of Platanista. The 

 parietal appears in the shape of a wide arch, receiving in its concavity the squamosal, 

 and articulating for a space of -6" with the well-developed alisphenoid, thus completely 



