34 THE VOYAGE OF H.M.S. CHALLENGER. 



medius. According to Macalister 1 the two muscles are hardly separable in the 

 W( mibat and Sarcophilus. As a general rule, however, in Marsupialia they are quite 

 distinct. It is a wide thin muscle which arises in both instances from the dorsum ilii 

 and from the ischium as far back as the gemelli. It thus almost completely covers the 

 superior aspect of the hip-joint (PL III. fig. 1, en.g., and PL IV. fig. 5, en.g.). In the 

 discus its anterior border is blended with the corresponding margin of the gluteus 

 maximus. It narrows towards its insertion, and all its fibres converge to be attached to 

 the anterior border of the great trochanter of the femur. 



Gluteus quartus. — In Thylacinus this muscle is well developed (PL IV. fig. 2, g.q.). 

 Somewhat triangular in shape it springs by a pointed origin from the margin of the 

 acetabulum close to the reflected tendon of the rectus femoris (r.f 2 .). Expanding as it 

 proceeds downwards over the anterior aspect of the capsule of the hip-joint, it enters the 

 interval between the vastus externus and vastus internus, and is inserted into the front 

 of the shaft of the femur immediately below the neck. In the Cuscus the gluteus quartus 

 is also present, but it is very feebly marked, and consists of a single delicate muscular slip. 



In neither case was I able to make out with precision the nerve supply. Until this 

 is done the homologies of this minute muscle cannot be considered as established. Although 

 usually grouped with the gluteal muscles it seems to have a closer relationship to the 

 extensor muscles. It is generally present in Marsupiala. Macalister 1 describes it in 

 the Wombat, Sarcoplvilus, Giant Kangaroo, Bennett's Kangaroo, the Phalanger, and the 

 Opossum. Neither Macalister 2 nor Young 3 mention it as existing in the Koala. 



Pyriformis. — This is a strongly-marked triangular muscle, which lies upon the posterior 

 part of the gluteus minimus behind the gluteus medius (PL III. fig. 1, p. ; and PL IV. 

 fig. 5, p.). It springs from the lower surface and outer margin of the sacrum, and is 

 inserted by a rounded tendon into the posterior part of the summit of the great troch- 

 anter. It is identical in both animals. Macalister 1 states that it is " undistinguishable 

 from the gluteus medius in the Phalangista." 



Ischiofemoral (PL III. fig. 1, if., and PL IV. fig. 5, if.).- — This muscle is commonly 

 described as the quadratus femoris, but it lies on a different plane, viz., superficial to 

 the gemelli, the true quadratus and the adductor magnus. It seems to me therefore 

 that it must be regarded as a separate muscle, the precise nature of which is doubtful. 

 It is an exceedingly thin fleshy band which arises from the great sacro-sciatic ligament, 

 and is inserted into the posterior aspect of the femur below the great trochanter between 

 the insertions of the ecto-gluteus and the adductor magnus. It is more strongly 

 marked in the Thylacine than in the Cuscus, and its deep surface is partially fused with 

 the subjacent adductor magnus. 



1 Myology of the Wombat and Tasmanian Devil, Ann. and Mag. Nat. Hist., vol. v., 4th series, p. 167. 

 - Muscular Anatomy of Koala, Ann. and Mag. Nat. Hist., vol. x., 4th series. 

 3 Young, Anatomy of Koala, Jour, of Anat. and Phys., vol. xvi. 



