1901,] LARTXX OF CEBTAIX WHALES. 29l 



arytenoid cartilages, and below the articular facets the cricoid 

 passes as a nearly horizontal band, much narrower than before, 

 round the side and across the ventral surface to the other side. 

 At the lower angle formed by the lateral and dorsal moieties is 

 the facet for the thyroid corjiu, and from here a slight ridge passes 

 obliquely forwards and ventral wards to reach the anterior margin 

 of the latero-ventral moiety of the cricoid ; this is much thinner 

 and of less diameter than the dorsal moiety. 



The measurements of the cricoid are as follows : — 



Length along the median dorsal ridge 1| inches. 

 Width of each limb of V 1 i inches. 

 Separation of thyroid facets 2 inches. 

 Width of the latero-ventral moiety | inch 

 (except in ventral mid-line, where it is | inch). 



In the Pilot Whale the cricoid has a much greater posterior 

 (dorsal) surface, for according to Murie (1871) it is a " trifle 

 longer than the body of the thyroid," while on the ventral surface 

 it is produced into completely posterior cornua which embrace 

 the trachea and " wellnigh meet in the mid-line." 



In the Porpoise, too, the cartilage is incomplete ventrally 

 (Owen). 



The arytenoid cartilage (PL XXVII. fig. 16) is of considerable 

 length, as in other Odontocetes, and though the same regions may 

 be recognized as in the arytenoid of Balcenoptera, these are less 

 distinctly marked off from one another. But the descriptive terms 

 used in that Whale are no longer appropriate here. In general 

 form it closely resembles the corresponding cartilage in the 

 Porpoise. 



Each arytenoid is a long flat rod, oval in transverse section 

 through the greater part of its extent, broader and thicker 

 inferiorly, thin and flat superiorly. On the dorsal side is a distinct 

 shoulder, at a point about two-thirds of its length from the summit; 

 this makes nearly a right angle with the narrow (superior) moiety, 

 and from this point, which projects about k inch, the dorsal 

 (or posterior) border slopes very gradually downwards ; on this 

 margin, just below^ the shoulder, is the articular facet from the 

 cricoid. 



The lower end of the cartilage is broad and rounded and very 

 thick. 



The upper moiety or supra-arytenoid (Thompson) becomes quite 

 thin as the extremity is approached, and the plane of this narrow 

 plate becomes twisted near the extremity, so as to take on a 

 position finally which makes an angle with the plane of the broad 

 face lower down. The margin of this upper extremity is recurved, 

 and supports the mucous membrane that constitutes the arytenoid 

 body, and the two cartilages touch one another here. 



The external surface of the posterior moiety or " processus 

 vocalis " is irregularly convex and serves for the attachment of 

 muscles. 



