1926] Hall: Skull of the Bodent Otospermophilus grammurus heecheyi 381 



width 3 mm. over that of the normally shaped interparietal of this age. 

 The general shape changes from the oval pattern of A and B (see fig. 6) 

 to the square one of / shown in figure 3. 



Tympanies. — Only a very narrow half -ring of ossification is present 

 in A. This ring lies in a horizontal plane. Membrane extends both 

 laterally over the otic region to the external auditory meatus and 

 postero-ventrallj' to the junction of the petrosal, basisphenoid and 

 basioccipital. Ossification proceeds in this membrane as shown in 

 figures 17-20. From figure 26 it will be seen that the tympanic lies 



Fig. 26. Showing relations of auditory ossir-les and related structures in an 

 embryo of 39 mm. from the same litter as B. Greatly enlarged. 



Explanation: ang., angle of mandible; ccr., coronoid process; inc., incus; 

 mal., malleus; mc, Meckel's cartilage; pet., petrous; sq., squamosal (overlying 

 the end of Meckel's cartilage and a part of the incus); st., stapes; tym., 

 tympanic ring (anteriorly spreading out over Meckel's cartilage and a portion 

 of the malleus, posteriorly covering the stapedial process of the incus). 



against Meckel 's cartilage and that a part of the tympanic is extended 

 posteriorly on the inside of the cartilaginous mallevis and another 

 smaller part anteriorly along Meckel's cartilage. The presence of a 

 small element slightly in front of the anterior end of the tympanic 

 ring in C, but not noted in any other stage, suggests the possibility that 

 all, or at least part of this portion of the tympanic that lies spread over 

 Meckel's cartilage represents the goniale. Careful scrutiny, however, 

 fails to reveal any separation between the major portion of the 

 tympanic ring and the part spread over Meckel's cartilage. It is 

 possible that a slightly later stage would reveal a separation of these 

 two parts. However, at this time, in C, the malleus and incus, although 

 still adhering to Meckel's cartilage, are clearly defined from it and 

 from each other. No separate element that could be interpreted as a 

 goniale, as was doubtfully done with the element in C, was found in 

 the stage shown in figure 26. Except for the spreading of the tympanic 

 over Meckel's cartilage, this relation of the tympanic to Meckel's 

 cartilage does not differ greatly from the relations in some other forms, 

 judging from the figures given by Parker (1885) of Dasypus (pi. 7, 



