CHELONIA. 35 



and in Chel. imbricata especially the homologous apertures are twice as long as they 

 are broad. The pterygoids, in the Chel. cuneiceps, develope a sharp ridge along their 

 median suture ; and short but well-defined processes at their anterior and outer angles. 

 The channel or concavity upon the under part of the diverging portion of the 

 pterygoid conducts obliquely into the temporal fossa? in the Chel. my das ; in Chel. 

 cuneiceps it leads directly forwards upon the under surface of the anterior part of the 

 pterygoids exclusively, as in the Chel. imbricata and Chel. caouanna. 



In the Chel. mydas the malar approaches the mastoid very closely, and sometimes 

 touches it by the posterior angle, thus separating the squamosal from the postfrontal ; 

 the extent of the union between the squamosal and postfrontal is also shorter in the 

 Chel. caouanna than in the Chel. imbricata. In the extent of that union (between 12 

 and 27) the Chel. ctmeiceps resembles the Chel. imbricata, as do likewise the Chel. 

 breviceps and Chel. longiceps. But the Chel. cuneiceps differs from all the recent species 

 in the form of the squamosal (27), which is bent upon itself, forming a slightly curved 

 linear eminence, where the lower and smoother part of the bone is bent, and, as it 

 were, pressed inwards towards the tympanic (28), against which it abuts. This 

 modification is natural, not the effect of accidental pressure upon the fossil. The 

 lower border of the malar (26), which intervenes between the maxillary and squamosal, 

 is sharp but convex, as in Chel. caouanna, not concave as in Chel. mydas, nor nearly 

 straight, as in Chel. imbricata. But the concave curve of the inferior margin of the 

 squamosal (27) most resembles that in Chel. imbricata. The antero-posterior extent 

 of the mastoid (8) is less proportionally than in any of the recent Chelones, and it 

 forms a smaller share of the upper border of the large meatus auditorius. The articular 

 part of the tympanic descends below the squamosal further than in the recent turtles ; 

 and its articular surface is more convex at its outer half, and more concave at its 

 inner half; Chel. imbricata makes the nearest approach to the fossil in this respect. 

 In the Chel. mydas and Chel. caouanna the articular surface is nearly flat. 



As the supracranial scutella have left unusually deep and well-marked impressions 

 on this fossil skull, I have reserved their description, and the comparison of their 

 different forms and proportions in the several fossil species, to this place. 



Three scutella occupy the median line of the upper surface of the cranium in the 

 present species of Chelone, which, from the absence of any impression along the 

 frontal and sagittal sutures, appear to have been single and symmetrical. The 

 anterior and smallest answers to the " frontal" scute (fr) ; the next in size and position 

 to the " sincipital" scute (sy) ; the hindmost and largest answers to the " occipital" 

 scute (oc), which is usually divided, and forms a pair in existing Chelones. 



The frontal scute is long, narrow, hexagonal, broadest across the antero-lateral 

 angles, from which the impressions extend outwards to the supraorbital margin, 

 which divide the " fronto-nasal" scute from the "supraorbital" scute (ob). 



The sincipital scute is bounded on each side by a sigmoid curve, and both before 



