378 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



abaxial, face of the corresponding radial. The axial face 

 is divided transversely by a well-marked ridge into two 

 portions of unequal size. The larger portion is on the 

 oral, or upper, side of the ridge, and is sub-divided by a 

 median vertical and two oblique and less prominent ridges 

 into two pairs of fossae, of which the upper pair are the 

 deeper. The smaller portion of the face is on the aboral, 

 or lower, side of the ridge, and is wholly occupied by a 

 broad and deep fossa. At the point of intersection of the 

 transverse, vertical, and oblique ridges is the oval opening 

 of a canal which traverses the plate from face to 

 face. The smaller abaxial face (fig. 9) is of simpler 

 character, and presents only one pair of fossae, divided by 

 a vertical ridge which passes round the opening of the 

 above-mentioned canal, and at its upper, or oral, end is 

 continuous with a slightly raised margin, which bounds 

 the fossa? above. The oral margin of the plate presents a 

 well-marked notch when viewed from this face. 



Second Primibrachial Plates. — These plates resemble 

 the radials in general form (PL I., fig. 3, pmb. 2; figs. 10 

 and 11). Each one presents a triangular figure when 

 viewed from above or below, and has three articular faces. 

 Of these, the axial closely resembles the abaxial face of 

 the first primibrachial, with which it is articulated, having 

 a single pair of lateral fossae divided by a vertical ridge, 

 which also passes round the transversely oval aperture 

 of the axial canal. The surfaces of the lateral fossae slope 

 away slightly from the vertical ridge, so that when the 

 latter is in close contact with the corresponding ridge of 

 the first primibrachial the second primibrachial has a 

 slight range of lateral movement upon the first. A 

 median crest and a pair of divergent lamellae project from 

 the upper, or oral, margin of the axial face to form parts 

 of the two oblique surfaces of articulation (fig. 11). 



