1889.] NATURAL SCIENCES OF PHILADELPHIA. 21 



the rudimentary interhyal in the tendon of the stapedius muscle, but 

 might do so were additional material at hand. These wood rats have 

 tympanic bullae of no inconsiderable size, and in the dried skull of 

 an old A 7 , dnerea their lower surfaces are quite transparent, each 

 being somewhat laterally compressed and inclined towards each 

 other, so that were their imaginary horizontal long axes produced 

 to the front they would intersect at a point just slightly anterior to 

 the posterior narial aperture. 



In form, then, these bullae are subcompressed ovoid, where mesi- 

 al ly they are moulded upon the bones they come in contact with 

 at the base of the cranium at its infero-external aspect, which have 

 to do with the auditory chamber. 



Turning for the moment from our consideration of the proper 

 " investing bones," I desire to pass a few remarks upon the " hyoi- 

 dean apparatus." Essentially, this seems to be built up upon the 

 plan of these parts as we find them in the more highly organized 

 eutherian mammalia generally. In the specimen before us, however, 

 ossification even at this stage appears but to have advanced slowly, 

 the several segments of the arch yet being largely in cartilage. 



All the elements of these parts seem to be present in this embryo, 

 and the most notable feature to me is the form of the basi-branchial 

 as I make it out. It is far more extensively curved than Parker 

 found it to be in the hedgediog, by which I mean the curved rod of 

 cartilage composing it is longer (Plate I, b. h. br.). By examining 

 the arch in an adult N. cinerea, the same feature seems to be present, 

 for the " body of the hyoid " is there larger and curved. Another 

 mammal, the dog, as drawn by Flower, also shows a curved basi- 

 branchial, whereas in the armadillo it is a medium piece united 

 apparently with the thyrohyals (Parker). The thyrohyals in the 

 embryo Neotoma are but feebly developed (t. hy), and remain in- 

 conspicuous after ossification in them is completed. Agreeing with 

 most ordinary mammals the three remaining pieces of the hyoid 

 present nothing of marked peculiarity. 



Returning to the investing bones of the skull, there yet remains 

 two of them to be described in the present connection, viz. : — the 

 palatine and the pterygoid. 



If there be any superficial ossific deposit yet in either pterygoid 

 (Plate I, Fig. 3, pg) the writer failed to discover it after a very 

 careful investigation. We are aware that in the pig at the fourth 

 stage of its development the pterygoids are somewhat advanced in 



