184 ON THE MORPHOLOGICAL CONSTITUTION OF 



This double piece, or pair of bones, presents the relations of 

 the interparietal bones in lower Mammalia. They may extend 

 laterally to join the " mastoids," or they may be connected to 

 the latter by a more or less continuous chain of " triquetral 

 bones" in the line of the lambdoidal suture. 



The zygopophyseal attachments of the " petrous portions 

 of the temporal bone " indicate these masses to be neurapo- 

 physes enveloping the ossified auditory capsules. Keeping 

 out of view the " squamous," " tympanic," and " styloid" por- 

 tions of the "temporal bones," the "mastoidal portions" 

 become early and intimately connected with the "petrous 

 portions." Commencing with the "petrous portions," as an 

 inferior pair of temporal neurapophyses, they are surmounted, 

 as in the lower Vertebrata, by the "mastoidal portions" as a 

 second pair of neurapophyses, while the arch is closed by the 

 doiible element which forms the upper angle of the " occipital 

 bone," as a meta-neurapophysis. There are well-marked indi- 

 cations of a temporal centrum in the human cranium. The 

 irregularly-truncated apices of the " petrous portions," directed 

 obliquely forwards and inwards, are continuous, by means of 

 the fibro-cartilaginous remains of the basis of the " primordial 

 cranium," which occupy the "foramina lacera media," with 

 the inclined plate of bone which, in the plane of the " basilar 

 process of the occipital" or occipital centrum, forms the back 

 part of the " body of the sphenoid," including the " posterior 

 clinoid processes." This plate of bone is frequently surrounded 

 by a deep groove, the posterior part of which lodges the 

 " transverse venous sinus," and I have seen it nearly detached. 



The feebly-developed post-frontals in the bird have dis- 

 appeared in the mammal, so that the post-sphenoidal centrum 

 is surmounted by the " all-sphenoids," as a single pair of neura- 

 pophyses ; and by the enormously- expanded double meta- 

 neurapophysis in the human subject, or the less developed 

 form of parietals in the Mammalia generally. 



