24 CALAMOCKINUS DIOMED.E. 



joints the reticulation appears quite coarse (Plate XIII. Fig. 15). See 'also 

 the coarse reticulation of the surface of the axillary in Plate XV. Figs. 6, 7. 



The distal faces of the axillary are greatly modified by the presence of 

 a sharp keel (Plate XV. Figs. 6-9) which separates its right and left faces, 

 the proximal faces of the first joint of the fork and of the arm fitting 

 ao-ainst the distal fiices of the axillary, having only one muscular fossa 

 and one for the dorsal ligament (Plate XV. figs. 9, 12), the other muscular 

 fossa being atrophied. In the case of the second axillary, the distal face 

 (Plate XIV. Fig. 12) shows the condition of the muscular fossae of the left 

 and right joints. 



The keel of the second axillary projects like a thin, short dagger-shaped 

 lamella between the first joints of the fork and of the main arm (Plate XIV. 

 FW. 12) ; it appears, when seen from the ventral side, as reduced to a mere 

 ridge extending from the centre of the upper triangular point of the axillary 

 to the centre of the lower face (Plate XIV. Fig. 11). 



The }Oung arm joints near the extremity of the arms differ entirely in 

 their structure from the older joints with their complicated sutural joints, 

 broad lateral processes, and high dorsal part of the arm joint, and the 

 insertion pit for the pinnule. An arm joint, the fifth from the end of the 

 arm (Plate XII. Fig. 8), still shows plainly the simple original reticulation 

 arranged in radiating lines towards the articulating faces, and forming an 

 open network of large irregularly shaped cells. Some of the shanks of 

 the newly formed cells remain as large curved spines on the anterior ex- 

 tremity of the dorsal part of the arm joint. A slightly older joint (Plate 

 XII. Fig. 9) shows a greater extent of the closer radiating reticulation ad- 

 joining the sutures of the joints. In this joint the axial canal is formed 

 by the bridging over of the concave floor of the ventral side of the joint 

 by spurs running out laterally from the large cells in the central part of 

 the arm joint, until they meet from opposite sides and form a thin floor 

 of large-meshed cells enclosing the continuation of the axial canal (Plate 

 XII. Fig. 10). A profile view of a still younger joint shows it to be merely 

 a curved sheet of open-work limestone reticulation (Plate XII. Fig. 11), and 

 the end view shows how rudimentary are still the sutural faces of the joint 

 (Plate XII. Fig. 12), although there is already a marked indication of the 

 difference in size so prominent in the older arm joints between the opposite 

 sides of the joints enclosing the axial canal. The axial canal occupies a 

 comparatively large space in the sutural face of the arm joint. 



