320 



ANATOMY OF VERTEBRATES. 



posteriorly to form the narrow median nasal opening. The 

 roof is continued by the pterygoids, ib. 24 and 16^, which articu- 

 late, as in many Birds, with the tympanic, e, 28, and the basi- 

 sphenoid, 5. Another mark of ornithic affinity is the confluence 

 of the malar and squamosal, fig. 197, 27 : unless the slender pro- 

 cess of the maxillary, ib. 26, may represent the malar. The 

 tympanic cavity is excavated in the petromastoid and partly 

 closed by the slender tympanic, fig. 202, 28, e, which sends for- 

 ward a short homologue of the orbital process of that of the 

 bird: about three-fourths of the ear-drum are attached to the 

 tympanic, and one-fourth to the mastoid : the plane of the drum 

 is nearly horizontal and looks downward. The ' stapes ' is colu- 

 melliform, fig. 197, d\ one crus of the incus anchyloses with the 

 reduced tympanic at ; the other is confluent with the malleus, c. 

 The lower jaw consists in the Echidna, fig. 197, 29-32, of two 

 long and slender styliform rami without a symphysial joint, but 

 loosely connected together at their anterior extremities. An 

 angular process, so, divides the horizontal from the ascending 



204 



Skull of Ornithorhynchus, half natural size. Lxxxi'. 



ramus, which rises at an open angle and terminates in a small 

 oblong convex condyle, 29. A short obtuse coronoid process, 3i, 

 extends from the upper part of the horizontal ramus as far in 

 advance of the angle as the condyle is behind it. The rest of 

 the ramus is rounded like a rib, and diminishes to the anterior 

 extremity. The dental canal commences below the coronoid pro- 

 cess and divides in its progress, one branch terminating near the 



