380 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



surface the outer margins of the first secundibrachials 

 (PI. I., fig. 3) are seen to be considerably longer than the 

 inner, so much so that the angle of about 80° at which 

 their proximal articular faces incline towards each other 

 is widened out to one of about 130° between their distal 

 faces. The outer margin of the second joint also is longer 

 than the inner, and presents a shallow socket divided by 

 a transverse ridge, in which a minute perforation appears. 

 To this socket the first (oral) pinnule is articulated 

 (fig. 3). With certain exceptions to be presently described, 

 the succeeding joints present a similar inequality in the 

 length of their margins, but the longer margin is 

 alternately outer and inner, and the pinnule borne by 

 every such joint is articulated to it. Hence, when viewed 

 from the dorsal surface, the joints present the appearance 

 of triangles, the apices of which point alternately to one 

 side and the other (fig. 3). In the middle and more 

 distal parts of the arm the inequality of the margins 

 becomes less and less marked ; and while in the proximal 

 portion of the arm the articular socket of the pinnule 

 encroaches upon the distal articular socket of the segment 

 (fig. 14), in the middle and distal portions it is more 

 and more restricted to the lateral face (fig. 15). The 

 articular faces of the great majority of the secundi- 

 brachial segments present very similar characteristics 

 (figs. 12 to 16). A transverse ridge, pierced by the 

 opening of the axial canal, crosses each more or less 

 obliquely and separates a single deep fossa on its dorsal 

 side from a pair of shallower ventro-lateral fossae. A pair 

 of radial lamella? separates the latter from a second pair of 

 fossa?, which are deeper, occupy the ventral portion of the 

 articular face, and are themselves separated by a median 

 and more or less vertical lamella. The single dorsal fossa 

 lodges the elastic ligament which extends the arm; the 



