SkuU of Moclilorhinus platyceps. 167 



removing the matrix. There is no reason to anticipate the 

 recovery immediately of other parts of the skeleton. 



This head indicates one of the smaller Bidentalia (A. Gr. 

 Bain)j distinguished from many near allies by the singularly 

 smooth texture of the bone, which is almost as well preserved 

 as in specimens from English clays. The sutures are more 

 distinct than usual, and establish relations of some bones 

 which were previously less well shown. The head is a little 

 distorted, in consequence of contortion which has affected the 

 rocks. 



The genus Mochlorhinus is distinguished from Ptycho- 

 gnathus by three characters. First, the usual angular ridge 

 between the upper surface of the skull and face is wanting, 

 and the two areas, which commonly meet at an angle, graduate 

 into each other in this animal by a smooth rounded tran- 

 sition, convex from above downward, concave from side to side 

 (fig. 2). Secondly, the palate has the vomer elevated in frout 

 of the palato-nares, and the palatine bones at their sides so 

 as to form three prominent tubercles. Upon the summits of 

 these tubercles are minute teeth ; they are small, flat, and 

 circular on the summit of the vomer, but further back the 

 teeth become much smaller and pointed (fig. 3). Thirdly, the 

 head appears to be much more compressed from side to side 

 than is usual in Ptycliognathus^ in which the skull is relatively 

 wider in front of the orbits, giving the animals a table-headed 

 appearance between the orbits. With this character is asso- 

 ciated the position of the face at right angles to the crown of 

 the head, giving an unusual depth from the frontal region to 

 the palate, which exceeds the length from the nasal bones to 

 the interparietal by almost one half that distance (fig. 1). 



The genus is placed in the family Ptychognatharhinidai. 

 That group is characterized by the rapid elevation of the 

 brain as it extends forward and abuts against the interorbital 

 septum. 



From the occipital plate the head measures 4 inches in 

 length along the flat smooth crown to the rounded fronto- 

 nasal surface. In vertical depth the measurement is about 

 1\ inches from the frontal bone above the large lateral orbit 

 to the point where the tusks, descending vertically, emerge 

 through the maxillary bones. The roots of the tusks form 

 prominent vertical rounded ridges, k>\ inches long, at the 

 sides of the head; they originate a little below the orbits, at 

 less than 2 inches behind the vertical flattened prominence of 

 the intermaxillary which forms the median part of the face. 

 These lateral ridges give the face a transverse measurement 

 of about 4 inches. The tusks have a diameter of f^ inch at 



