SKELETON OF POLYPTERUS. 79 



ethmoid ; F, frontal ; Ma, mastoid ; M}}, metapterygoid ; Mx, maxillary ; 

 N, nasal ; 0, operculum ; Oc, occipital ; Pa, parietal ; PI, palatine ; Pmx, 

 prfemaxillary ; po, postorbital ; Prf, prefrontal ; Pt, post-temporals ; Ptf, 

 postfrontal ; Ptr, pterygoid ; Q, quadrate ; S, suspensorium ; So, suboper- 

 . culum ; Sp, sphenoid ; S2}1, splenial ; St, supratemporals ; T, tympanic 

 lamina ; Tu, turbinal ; v, vomer ; x x, small ossicles ; x' x', spiraculars. 



being greatly increased. Tliey are arranged much in the same 

 fashion as in Teleostei. But a great portion of the primordial 

 cranium remains cartilaginous. The membrane-bones which 

 cover the upper and lower surfaces of the brain-case are so much 

 developed as to cause the underlying cartilage to disappear, 

 so that a large vacuity or fontanelle exists in the substance 

 of the upper as well as lower cartilaginous wall. Of ossifica- 

 tions belonging to the primordial skull must be noticed the 

 single occipital with a mastoid on each side. They are sepa- 

 rated by persistent cartilage from the sphenoids and post- 

 frontals ; the former, which are the largest ossification of the 

 primordial cranium, enclose the anterior half of the brain 

 cavity. Finally, the nasal portion contains a median ethmoid 

 and a pair of prsefrontal bones. 



Only a very small portion of the bones described are 

 visible externally, nearly the whole of the primordial cranium 

 being covered by the membrane-bones. Of these are seen on 

 the upper surface a pair of parietals, frontals, " nasals," and 

 turbinals ; on the lower surface a large cross-shaped basal, 

 anteriorly bordered on each side by a pterygoid, parallel to a 

 palatine which forms a suture with the double vomer. The 

 suspensorium has in front a metapterygoid and quadrate 

 bone, and an operculum and suboperculum are attached to it 

 behind. 



Praemaxillaries and maxillaries are now fully developed, 

 but immovably attached to the skull. The lower jaw is ossi- 

 fied, and consists of an articulary, angular, dentary, and 

 splenial. Of labial cartilages a rudiment at the angle of the 

 mouth has remained persistent. 



