136 STRUCTURE AND CLASSIFICATION OF BIRDS 



1. THe bones formed by ossification from the chondro- 

 cranium are four occipitals, viz. basioccipital, two exoccipitals, 

 and the supraoccipital, forming a complete ring of bone 

 round the foramen magnum ; basisphenoid, with two wings, 

 the alisphenoids ; presphenoid, with two wings, the orbito- 

 sphenoids (occasionally atrophied, e.g. Apteryx) ; meseth- 

 moid, with two lateral wings, the ectethmoids (sometimes 

 termed prefrontals) ; these are occasionally absent as distinct 

 ossifications, and may sometimes, on the other hand, be very 

 large and even appear on the frontal surface of the skull, 

 marking the orbit anteriorly ; in those cases they take the 

 place of the orbital part of the lacrymal and have a better 

 claim to be called prefrontal. 



2. The investment of the auditory capsule, termed 

 collectively the periotic bone, consists of three separate 

 elements, the prootic, opisthotic, and the epiotic, which last 

 is absent in the Apteryx. 



3. The first visceral arch, the mandibular, has but two 

 bones ^ ossified from its cartilage, the quadrate and the 

 articulare of the lower jaw. The second and third arches 

 form the hyoid apparatus ; the ossifications are, first, the 

 columella auris, a bone corresponding physiologically, if not 

 also morphologically, to the ear bones of mammals ; secondly, 

 a median piece in front, cornposed of two fused pieces, the 

 basihyal, with sometimes lateral processes, the ceratohyals ; 

 thirdly, the basibranchial, with two long lateral outgrowths, 

 of :which the nearest regions are ossified to form the cerato- 

 branchials ; thirdly, a single median piece (sometimes absent)y 

 the urohyal, a remnant of the third arch.^ 



The membrane bones of the bird's skull are numerous, 

 and may be referred to the same categories as thfe cartilage' 

 bones. ■ ' ) 



1. Associated with the cartilaginous cranium are pos- 

 teriorly the parietals ; in front of these the firontals ; with' 

 the f rentals articulate the lacrymals, of varying development, ' 



■"' A mento-meckelian has been recently discovered in hawks. 

 ' For modifioatioua of hyoids see especially GADOW,'in Bronn's Thierrdch, - 

 and Gtetsel, Zeitschr. f. d. ges. Natiirw.tid.l^^^.-- ,■■ ...,.-.. (..!.;. 



