SKELETON OF POLYPTEKUS. 79 



ethmoid ; F, frontal ; Ma, mastoid ; Mp, metapterygoid ; Mx, maxillary ; 

 N, nasal ; 0, oijerculum ; Oc, occipital ; Fa, parietal ; PI, palatine ; Pmx, 

 prsemaxillary ; po, postorbital ; Prf, prefrontal ; Ft, post-temporals ; Ptf, 

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

 culum ; ;S^, sphenoid ; Spl, splenial ; St, supratemporals ; T, tympanic 

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



being greatly increased. They 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. Fiually, 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. 



Pr^maxillaries and maxiUaries 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. 



