158 Prof. A. Macalister on the Myology of 



a better-developed individual ; but this union does not seem 

 to be the invariable rule in Phascolomys. A union of the 

 origin of the subclavius with the insertion of the rectus abdo- 

 minis occurs in Orycteropus * ; and in this animal also the 

 muscle is a true sterno-scapularis, as also in the Porcupine. A 

 sterno-scapular band exists in tlie Llama, Rhinoceros, Hippo- 

 potamus, Axis, and other non-claviculate mammals ; but it is 

 interesting, as bearing on the homologies of this muscle, that, 

 except in a few rare cases as a human anomaly, it never coexists 

 with the ordinary subclavius. 



In Sarcophilus the subclavius passes from the first rib- 

 cartilage to the clavicle, but is not traceable further. In the 

 Virginian Opossum it runs from the first sterno-costal cartilage 

 to the outer third of the clavicle and the acromion process. 

 In the Phalanger its insertion is still more extensive, and in 

 Macropus gicjanteus and Bennettii its insertion extends for the 

 outer two-thirds of the clavicle. In none of these latter is its 

 sterno-scapular continuation marked. From the considerations 

 given above, I think we can scarcely regard the sterno-scapular 

 as any tiling but a variety of the subclavius. 



The rectus thoracicus arises from the lower part of the 

 sternum, as far as the summit of the mesosternum, by a thin 

 aponeurosis, which becomes fleshy and is inserted into the 

 second and third ribs external to their cartilages ; no fibres 

 arise from the sixth rib, nor are any inserted into the first. In 

 Sarco2)hilus the insertion is prolonged into the four upper ribs 

 from the sternum. This is the muscle which is considered by 

 Professor Rolleston (and, I think, with some reason) serially 

 continuous with the external oblique. I have called it rectus 

 thoracicus temporarily, for want of a better name ; but it is 

 evidently not the same as the more superficial rectus thoracis, 

 of Turner. 



The pectoralis quartus in theWombat and Sarcophilus covers 

 the side of the chest below the fifth rib, and is inserted into 

 the pectoral ridge of the humerus. In the Kangaroo and Wal- 

 laby this muscle is very large and superficial, its lowest fibres 

 blending with those of the panniculus carnosus, its hinder 

 fibres with those of the latissimus dorsi, and its anterior ones 

 with those of the great pectoral. It is smaller and more defi- 

 nite in the Phalanger, and most distinct and separate in the 

 Opossum. (For an account of the synonyms of this muscle, 

 see the Anatomy of Bradypus tridactylus, Ann. Nat. Hist. 

 July 1869.) Professor Owen regards this muscle as a dif- 

 ferentiated portion of the great pectoral ; and Prof. Humphry, 

 who has added another new name to the eight by which this 

 * Gallon, Trans. Linn. Soc. vol. xxvi. p. 572. 



