REPORT ON THE MARSUPIALIA. 35 



Obturator interims and gemelli. — The obturator interims is present in both the 

 Thyladne and Cuscus. It is strongly developed in the latter, but comparatively feeble 

 in the former. In both, its tendon plays in the small sciatic notch, but the encrusting 

 cartilage of this is not raised into ridges, and thus the tendon does not display the 

 longitudinal furrows which are so characteristic of the same muscle in man. It is in- 

 serted along with the obturator externus into the bottom of a deep recess which exists 

 between the great trochanter and the neck of the femur. 



The gemelli are enormously developed. In the Cuscus they are inseparable except at 

 their origin. The superior (or anterior) gemellus springs from the ischium in front 

 of the small sciatic notch, whilst the inferior (posterior) gemellus arises from the tuber 

 ischii. The tendon of the obturator internus lies in a groove in their substance, and 

 they are inserted into this tendon. The nerve to the quadratus femoris passes downwards 

 through the substance of the gemelli. 



In the Thyladne the gemelli are stdl more strongly marked, and each consists of a 

 superficial and deep segment. These fuse as they pass towards their insertion, which is 

 partly into the tendon of the obturator internus, but chiefly into the recess between the 

 great trochanter and the neck of the femur. The nerve to the quadratus femoris passes 

 downwards between the segments of the gemellus superior, but under cover of both parts 

 of the gemellus inferior. 



Obturator externus. — This is an exceedingly powerful fan-shaped muscle, which 

 presents the usual origin in both animals. It is inserted by a broad band-like tendon 

 into the trochanteric fossa below the tendon of the obturator internus. 



Quadratus femoris. — This is a thick fleshy muscle, which lies upon the same plane 

 as the adductor magnus, and is quite separate from it in the Cuscus, although in Thyladnus 

 it is partially fused with its upper border. It arises from the tuber ischii, and is inserted 

 into the lower part of the posterior border of the great trochanter, and also into the 

 posterior aspect of the shaft of the femur below this for nearly an inch. As compared 

 with the corresponding muscle in man, it is relatively larger, and its fibres are more 

 oblique and coarser. 



Hamstring Muscles. 



In both Thylacinus and Cuscus we find the following hamstring muscles: — (l) The 

 biceps with its accessory parts, (2) the semi-tendinosus, and (3) the semi-membranosus. 



Biceps. — This muscle presents the simplest arrangement in the Cuscus. It is com- 

 posed of two distinct parts, viz., an ischial part or biceps proper and a caudal part. 



The biceps proper (PL III. fig. 1, b.) arises by a narrow and pointed origin from the 

 ischial tuberosity in common with the semi-tendinosus. As it is followed downwards it 

 rapidly expands so as to assume a marked triangular form, and it is inserted into the 



