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The Pectoral Girdle (Text-fig. 3)
The pectoral girdle of Leiopelma has been described and graphically reconstructed by de Vos (1938 A). The reconstruction made, however,
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is not entirely typical, and it has been thought advisable to make a further description and illustration of the girdle.
Text-Fig. 3—Pectoral Girdle, ventral view.
The girdle has been flattened so that all parts are seen in the same plane. The functionally dorsal surface of the suprascapula has now become ventral. The procoracoid is shown as if it were largely of bone, but actually it remains cartilaginous until fairly late maturity.
cl, clavicle; old, dorsal margin of clavicle; clt, cleithrum; cor, coracoid; cpg, cartilago paraglenoidalis; epc. epicoracoid; fpc, fenestra between procoracoid and coracoid; fgl, glanoid foramen; lyg, “lymph gland”; proc, procoracoid; sc, scapula; ssc, suprascapula; st. sternum
The pectoral girdle is arciferous, but the method of overlapping (left over right or vice versa) is variable. At their extreme anterior, the two epicoracoids join to form a single piece of cartilage which is embedded in a characteristic bilobed mass of lymphoidal tissue called by Noble (1931) the “lymph gland”. Immediately surrounding the cartilage is a layer of dense fibrous connective tissue. Slightly further back the epicoracoids begin to separate, but for a short distance are joined ventrally by a narrow procartilaginous bridge. When overlapping begins, the latter gives way to fairly sparse fibrous tissue just sufficient to hold the two halves together.
The lateral extremity of the well-ossified coracoid on each side forms the postero-median boundary of the glenoid cavity. A strip of cartilage, the cartilago paraglenoidalis, separates the coracoid from the scapula. The latter, which encloses between its antero-median and postero-median divisions a glenoid foramen, reaches the clavicle antero-medially. Medially, also, it is adjacent to the procoracoid cartilage. The latter is very late in ossifying and in the few cases where ossification is at all definite it is restricted to the lateral region of the cartilage.
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Laterally, it is united with the scapula, but the line of fusion between the two bones is generally visible. The procoracoid retains a double head of cartilage abutting on the glenoid cavity. The medial boundary of the procoracoid ossification is inclined to be irregular.
The clavicle has two divisions, each in the form of a flattened, bony strip. The ventral division is wider than the dorsal. The two parts are united anteriorly to form a groove into which the anterior rim of the procoracoid cartilage fits. De Vos states that the clavicle fuses laterally with the scapula, but I have not been able to confirm this.
The suprascapula usually becomes calcified, but a strip of uncalcified cartilage is left between the suprascapula and the bony scapula. Like the clavicle, the cleithrum is formed of two fused strips, and again the functionally dorsal division is the narrower. The suprascapula fits into the groove between the two strips of bone.
The sternum has two posterior styles as in Bombina (de Villiers, 1922). These may become fairly heavily calcified (Text-figure 4A), but always retain an uncalcified medial connection between each other. As de Vos has shown, the medial, anteriorly projecting piece of cartilage really consists of dorsal and ventral elements, the former lying embedded in the M. sternohyoideus, while the latter lies below the two epicoracoids at their posterior border.