176 Prof. Eschricht on the Gangetic Dolphin. 



Micropteron according to Dumortier*; and I have shown (loc. 

 cit.) the existence of a similar hook in the Hyperoodon, prevent- 

 ing the petrous bone from falling off the cranium, even after it 

 has become quite loose by maceration. When I wrote the me- 

 moir, I was not aware of the above statement of Cuvier, respect- 

 ing the firm attachment of the petrous bone in our dolphin. On 

 examining the specimen, in which (fortunately for my present 

 inquiry) the tympanic bone had been knocked off from the pe- 

 trous portion on both sides, this Camperian hook of the Cachalot 

 showed itself so much developed, that for this reason alone the 

 petrous bone, though quite loosened, especially on one side, 

 could not possibly be detached without breaking the hook. In 

 the smaller cranium, the elongation, which is wedged in between 

 the temporal and occipital bones, does not quite extend to the 

 outer surface ; but in the larger specimen it appears to constitute 

 part, at least, of the very rough and uneven bony protuberance, 

 situated on the outside of the tympanic bone, and marked t in 

 PI. VII. fig. 1. In fig. 3. PI. VI. it is still more distinctly seen, 

 inclosed above by the temporal bone proper (/), externally by the 

 jugular process of the latter, and internally by the lateral process 

 of the occipital bone (o). 



The basilar part of the occipital bone (PI. VII. fig. lo) is placed 

 anteriorly, as in Cetacea in general ; only it is free, has the form 

 of a pointed wedge, and is foremost in the cleft of the posterior 

 margin of the vomer ; behind, it is situated between the two 

 pterygoid bones. It exhibits, however, the greatest breadth be- 

 tween the petrous parts of the temporal bones, strongly exca- 

 vated across (PI. VI. fig. 3, under the articulating surfaces k) in 

 such a way, that the lateral parts are curved like wings round 

 the bullae tympani, their obtuse margins being attached by 

 means of strong fibrous cartilage. Between the pterygoid bone, 

 the pars petrosa, and the temporal bone is formed the great aper- 

 ture, which answers partly to the canalis caroticus and to the 

 foramen lacerum anticum, but is otherwise connected with the 

 large cleft which is prolonged anteriorly between the plates of 

 the pterygoid bone. Pursuing this large aperture along the ex- 

 terior margin of the bulla? tympani, a firm connexion is dis- 

 covered with that surface of the temporal bone, which runs par- 

 allel with the pterygoid bones, and which, quite externally, at 

 the point of transition into the root of the zygomatic process, is 

 converted into a deep, round-oblong cavity, exactly behind the 



avec des notes par M. G. Cuvier. Paris, 1820, 4to, p. 108, Atlas in fol. 

 transv. t. 22-25, 



* "Memoire sur le Delphinorhynque micropore echoue a Ostende," 

 Mem. de l'Academie Royale de Bruxelles, torn. xii. 1839, 4to, p. 10. 



