20 PROCEEDINGS OF THE ACADEMY OF [1889. 



two halves here being held in juxtaposition by the not inextensive 

 union between the fore ends of the Meckelian cartilaginous rod. 

 Those toothsacs for the lower incisor teeth are about as far advanced 

 as those described for the preinaxillary bones, and this applies with 

 equal truth to the molars of this lower jaw, though perhaps these are 

 not quite as far advanced as the molars of the maxillary above. 



For the most part the mandible has to no small degree ossified. 

 We are to note, however, that the summit of the low and ill-defined 

 "coronoid process " is tipped with nascent cartilage, and this state 

 obtains also with the yet growing condyle, here harbored in the shal- 

 low glenoid cavity of the squamosal on either side. Below these 

 protuberances a prominent angular process is to be observed, also 

 cartilage-tipped as in the case of the condyle and coronoid. These 

 several prominences of the posterior end of either ramus of the lower 

 jaw in this embryo rodent, gives it a very marked vertical depth and 

 a fan-like form, which renders it quite conspicuous. Through the 

 ramus and thoroughly ensheathed by it, longitudinally courses the 

 slender Meckelian rod of cartilage, to fuse anteriorly, as already 

 stated, with its fellow of the opposite side. 



At its hinder and free end, the Meckelian cartilage becomes clubbed 

 and of a peculiar form ; the extremity proper is cupped to receive 

 in articulation the incus, while below this cup the rod sends forwards 

 and downwards a very delicate, cylindrical spur of cartilage, here 

 incorporated in the plane of the membrana tympani, within the semi- 

 arc of the tympanic annulus. These parts seem yet to be largely 

 performed in cartilage, and we see the malleus in the hinder end of 

 this Meckelian cartilage, while the labors of Salensky, Fraser and 

 Kitchen Parker seem to have at last definitely decided that " the 

 incus is the upper element of the first or mandibular arch." 

 Here in this embryo Neotoma, the incus develops an unusually long 

 posterior crus, as shown in Figure 1, extending over towards the 

 auditory bulla (an). As in the vast majority of the higher groups 

 of the mammalia, the stapes is stirrup-shaped, and its foot-piece 

 closes up the fenestra ovalis, and this stapes according to the most 

 recent researches has been said to correspond with the hyomandibu- 

 lar of fishes, or in other words is the upper element of the hyoid arch, 

 as the incus, as we have just said, is the off-constricted piece of the 

 proximal extremity of the mandibular arch. Agreeing with the 

 first two described auditory ossicles, the stapes at this stage in Neo- 

 toma fuscipes, seems yet to be wholly in cartilage. I failed to detect 



