146 MR. W. K. PARKER ON THE STRUCTURE 



mass lies on a shelving wall, the postpituitary wall, a scooped and slanting tract of bone, 

 split at the mid line, from which the notochord has retreated. This fissure is the 

 remains of the posterior basicranial fontanelle of the Reptile. 



The prepituitary region has the anterior clinoid wall reaching the bony alisphenoid 

 (als), and a pinched front portion, which forms the postero-inferior angle of the inter- 

 orbital plate, and which afterwards coalesces with the huge perpendicular ethmoid below 

 the presphenoid. 



This great mesethmoidal plate has increased in size, and is growing into the frontal 

 region. The frontals themselves at present fail to wall in the space caused by the 

 abortion of the orbito-sphenoids in the chondrocranium ; there is, therefore, a post- 

 orbital fontanelle (po). 



The subfalcate squamosal helps the alisphenoid (sq, als) to finish the side walls ; it 

 clamps the prootic, overlies the alisphenoid, reaches the frontal, and supplements the 

 parietal (p) *. 



Skull of Gavia ridibunda. 3rd stage. 



This stage, in pulli a few days older than the last, has been worked out principally to 

 display the exquisite architecture of the great skull-floor, including the occipital, tem- 

 poral, and posterior sphenoidal regions. 



At this stage we see best this most curious piece of morphological mosaic, not quite 

 unlike the wall-work of the test of an JEchinus, but of a more complex type by far, and 

 yet having fewer parts as far as mere number goes. 



Indeed two very distinct strata are condensed and calcified to produce these results ; 

 and this difference is indicated in the figures by the colouring of the inner stratum, 

 formed by ossification of the chondrocranium, and the outer, which is the inner stratum 

 of the submucous connective web, is left uncoloured. 



The first thing that strikes the eye in the under view is the long bony dagger and the 

 deltoid bone which lies beneath and behind it. These divisions of the ichthyic para- 

 sphenoid are the rostrum and the basitemporals (Plate XXVII. figs. 1-3, pas, bt) ; on each 

 side the rostrum widens between the basipterygoid cartilages (bpg), which are lessening ; 

 at the mid line it is scooped correlative to the meeting together of the right and left 

 Eustachian tubes (eu), and behind it simulates the basitemporal wings, widening and 

 spreading. 



These hinder wings form the " anterior tympanic recess " (atr, ppg) : they are long, 

 falcate, notched, and grooved plates of bone, and with the help of the underlying 

 deltoid plate, the coalesced basitemporals (bt), they form a pair of very elegant bony 

 trumpets leading into the drum of the ear, and are a specialization of the first visceral 



* One of the older antomists (Owen), not seeing the squamosal in the inside of the struthious skull, took one of 

 his remarkable intellectual leaps, and supposed that it was always absent from that part, a hard thing to find in any 

 of the " Carinatee." 



This great theorist wanted the bone on the outside and further down for transcendental uses ; it, however, refuses 

 to leave its own supratemporal region, for it must be ready to graft itself on the auditory mass in all the hot-blooded 

 Vertebrata. 



