1889.] NATURAL SCIENCES OF PHILADELPHIA. 17 



form of a lune, its straight edge being directed anteriorly, and the 

 middle third of the same forming the hinder boundary for the 

 "fontanelle," while its arc behind is juxtaposed to the non-united 

 supraoccipitals, the extremities of the bone jutting at either lateral 

 aspect into the unossified area above the auditory bulla. In an 

 adult Neotoma cinerea the interparietal can be easily made out by 

 its sutural margins, and it will be seen that it comes there more to 

 contribute to the cranial roof, rather than to close in the back of the 

 brain-case as it does in our embryo N.fuscipes. 



Of all the investing bones of the outer surface of the cranium in 

 our specimen none possess a greater interest than the squamosal. 

 One of these (sq) stands between the frontal and parietal of the same 

 side above, and the primary structures of the ear and associate parts 

 below. Essentially, at this stage the squamosal is a flat, bone of a 

 pear-shaped outline, the smaller part being directed backwards, and 

 the larger oval moiety forwards, which latter bears near its anterior 

 periphery the forward-projecting zygomatic process. It articulates 

 with the posterior extremity of the jugal of the corresponding side. 

 Even in the adult Xeotoma, the glenoid facette of the squamosal is 

 not nearly as definitely marked as we find it in many other forms of 

 eutherian mammals, so in this embryo the feature is still less pro- 

 nounced at the stage of development in the specimen before us. In 

 figure 1 of my plates I have intentionally slightly dislodged the 

 mandible from its normal position the better to show the embryonic 

 otic structures, which latter have to a less degree been similarly 

 dealt with. 



Apparently one of the scale-like, small, and fairly well ossified 

 lacrymals presents a greater surface to the facial aspect of the skull 

 than it does to the orbital, and at this stage the lacrymal canal 

 (Plate I, Fig. 1, I. c.) may be seen just anterior to the slit-like and 

 extensive infra-orbital foramen. 



Professor Flower has said that this orifice "is always well within 

 the margin of the orbit," 1 but this wood rat undoubtedlv constitutes 

 an exception to the rule, for in the embryo it is found as I have just 

 described it, and in the skulls of the adult specimens of N. cinerea 

 at my hand, the lacrymal canal is found within that vertical fissure 

 which in these rodents represents the infra-orbital foramen. It 

 is here quite large, wedge-shaped, the edge being below, and its 

 roof above formed by its maxillary, which base also forms the verti- 



1 Flower, W. H. Osteology of the Mammalia, 3d Edition, p. 181. 



