OF DIDELPIIYS VIRGINIANA. 53 



dicular pieces, with the neur- and par-apophysis and neural spine of the parietal, and neu- 

 ral spine of the occipital, vertebra. So the nasal hajmapophysis connects mediately or 

 immediately, its own with each of the three other cranial vertebrae ; such extensive con- 

 nections not only contribute to the strength and fixity of the palato-maxillary arch itself, 

 but of the whole skull. The haimal spine is permanently double ; it maintains distinctness 

 from the hremapophysis ; above, the apex of the neural spine is wedged in between its 

 lateral moieties ; below and anteriorly, these are in mutual apposition, thus closing in the 

 termination of the neural canal. The " diverging appendage" of the palato-maxillary 

 arch (pterygoid), a delicate scale-like bone, loosely attached, extends backward under the 

 centra of the frontal and parietal arches ; it fiills far short of the frontal pleurapophysis, 

 instead of connecting the latter with the nasal pleurapophysis, as in birds ; nor is it sub- 

 servient in any way to either the stability or the mobility of cranial segments. The or- 

 bital dermo-skeletal, and the acoustic and olfactory splancho-skeletal, bones are elsewhere 

 noticed : the opthalmic sense-capsule is unossified. 



h. Of the Several Cranial Bones. 



Most of the cranial bones of this low mammal retain their individuality ; a few others 

 only coossify with age ; several are confluent from an early period of their formation, if 

 not originally connate. The skull is readily disarticulated, and reducible to a greater 

 number of pieces than is the case with higher mammals. Nearly all the bones afford in- 

 structive evidence of their morphological characters. The comparatively few compound 

 bones result from confluence of but few morphologically distinct elements : there is no 

 such complex " bone" as, e. gf., the "sphenoid" or " temporal" of anthropotomy. The 

 mammalian modification of cranial vertebrfc may therefore be studied under advantageous 

 circumstances. 



The skull ordinarily falls into the following pieces, some of which are simple, and others 

 compound, bones ; the latter are in italics : — occipital ; superoccipital ; two petromastoids ; 

 parietal, either single or double; tympanic (with ossicula auditus attached) ; sphenoid; 

 vomer with ethmo-turhinal ; frontal, single or double ; two nasals ; two premaxillaries ; two 

 maxillaries ; two palatals ; two pterygoids ; two malars ; two squamosals ; two lachry- 

 mals ; and the mandible (qu. italics), in halves. Sometimes the great sagittal crest comes 

 off the parietals, having only sutural union, and being apparently developed in the fascia 

 between the temporales. The superoccipital may unite with the parietals in old age. 

 The interparietal suture is often obliterated ; the interfrontal usually. There are no pre- 

 or post-frontals, nor stylo-hyals ; unless the vertical plate of the ethmoid represents the 

 first named. I have not observed an interparietal, or Wormian bones. Ordinarily there 

 is nothing to indicate that the mandible is formed of articular, angular, splenial, coronal, 

 and dentary pieces ; but its symphysis is imperfect, being only a synchondrosis, like the 

 pubic and sacro-iliac. 



The following are the commonly observed confluences of originally distinct bones : — Ex- 

 and par- with basi-occlpital, producing an "os occipitis" the spine of which remains sepa- 

 rate ; its neurapophyses meet above the foramen, but do not ordinarily coalesce. Basi-, 

 ali-, pre- and orbito-sphenoid, resulting in an " os sphenoidale" which is not further compli- 

 cated by coossification with the pterygoids. Mastoid with the otic sense-capsule (petrosal) 

 forming the " petromastoid" ; the separation of these elements from the squamosal and 

 tympanicf and the absence of a stylo-hyal, reduces the ordinarily complex " os temporis " 



UUMOIBS B08T. SOC. NAT. H18T. VOL. H. 14 



