146 ON THE MORPHOLOGICAL CONSTITUTION OF 



bones into the anterior and external portion of this fibrous 

 layer, the orbit may be more or less shut off from the temporal 

 fossa. 



The cartilaginous laminae which support the eyelids of the 

 mammal are developed in the fibrous layer which constitutes 

 the operculum of the orbit, and lie in the same morphological 

 plane as the malar and lachrymal bones. Their histologies!, 

 as well as morphological relations appear to me to indicate 

 not only that the palpebral cartilages are actinal elements of 

 the endo-sclerome, but also that they are anterior or external 

 hasmactinapophyses of the ethmoidal sclerotome. This view 

 of the morphological relations of the malar bone, palpebral 

 cartilages, and opercular membrane of the orbit in the mam- 

 mal, is borne out by the corresponding arrangement in the 

 bird. A fibrous membrane extends backwards over the orbit, 

 from the posterior extremity of the feebly-developed'maxillary, 

 and from the posterior margin of the descending process of 

 the ethmoido-frontal. In the lower part of this membrane the 

 malar is imbedded ; across its centre the palpebral cartilage ; 

 and at the antero-superior angle of the orbit, the lachrymal 

 bone. These have all distinct actinapophyseal characters, 

 which, in the case of the lachrymal, enables us to perceive 

 more clearly how the mammalian lachrymal, having become 

 intercalated between its corresponding hsemapophysis and 

 neurapophysis, retains only so much of its actinapophyseal 

 character as is indicated in the anterior margin of its groove, 

 the remainder of the bone being a secondary expansion. 



The lachrymal bone of the bird may extend into the 

 orbital membrane along the outer margin of the so-called 

 " principal frontal," or sphenoido-frontal, and become attached 

 to that bone without losing its connection with the ethmoido- 

 frontal. It may thus also form a union with the supra-orbital 

 bone, when that bone is present, as in the hawks. The 

 lachrymal may, moreover, extend backwards under the eye to 



