. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. THE SKELETON OF THE MAMMAL-LIKE REPTILE CISTECEPHALUS 237 1 cm. P Fig. 18. Cistecephalus sp. BPI 4086. A. Left astragalo-calcaneum in proximal (dorsal) view. B. Left pes in ventral view. straight down to a second support at the anterior edge of the procoracoid. This may be associated with the forward position of the shoulder girdle as a whole relative to the vertebral column and skull. Besides being strongly braced by scapulae and procoracoids, the clavicles also meet in the ventral mid-line in an ex
. Annals of the South African Museum = Annale van die Suid-Afrikaanse Museum. Natural history. THE SKELETON OF THE MAMMAL-LIKE REPTILE CISTECEPHALUS 237 1 cm. P Fig. 18. Cistecephalus sp. BPI 4086. A. Left astragalo-calcaneum in proximal (dorsal) view. B. Left pes in ventral view. straight down to a second support at the anterior edge of the procoracoid. This may be associated with the forward position of the shoulder girdle as a whole relative to the vertebral column and skull. Besides being strongly braced by scapulae and procoracoids, the clavicles also meet in the ventral mid-line in an extensive face-to-face contact. These features of the pectoral girdle can be interpreted in terms of resistance to strong lateral forces which were imposed upon the forwardly rotated glenoid region of the scapulocoracoid. Such forces would have been transmitted internally on to the clavicle which, instead of bracing the scapulocoracoid against stress from a posterolateral direction, had to support that bone against forces of almost purely lateral origin. To help counter these forces, the acromian articulation was extended dorsally so that the clavicle enjoyed an extensive dorsoventral overlap with the scapula; at the same time the ventral part of the clavicle rotated back to meet the anterior edge of the procoracoid in an extra point of support. The substantial meeting between the clavicles in the ventral mid-line may be regarded as a further strengthening device. The relationship between the glenoid cavity and humerus is also of signifi- cance. The glenoid is a well-finished socket, with a sharp dorsal rim on the supraglenoid buttress. The humerus carries a distinct humeral head, set off as a dorsal condyle on the proximal part of the bone, and it is evident that all movement between humerus and glenoid took place about its slightly convex articulatory surface. With the humeral head dorsally oriented the deltopectoral crest faces forwards, and the twist of the humerus along its
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectnaturalhistory, booky
