122 U.S. NATIONAL MUSEUM BULLETIN 2 73 



and in the same plane as contrabentes I and II. Robertson (1944) 

 describes it as branching off distalward into three heads: (1) One 

 joins the tendon of what appears to be a volar interosseous with 

 which it blends on the radial border of the dorsal extensor expansion 

 of digit V. This head sends a group of fibers to the radial head of 

 metacarpale V and may be regarded as a contrahens V with anomalous 

 attachments. (2) A second head joins an interosseous tendon which 

 meets the ulnar side of the dorsal extensor expansion on digit IV. 

 (3) The third head, which perhaps represents contrahens IV, joins 

 also an interosseous tendon to be continuous with the dorsal extensor 

 aponeurosis on the radial side of digit IV. This kind of contrahens 

 ending is, I repeat, peculiar. A full set of contrabentes seems to be 

 the rule in Cehus (see Forster, 1916b; Jouffroy and Lessertisseur, 

 1959; Straus, unpublished), but the pattern has been variously 

 described. Forster's observations (1916b) fit the type b) discussed 

 by Jouffroy and Lessertisseur (1959) and that of the French authors 

 corresponds to their a) model. Straus' findings (unpublished) appear 

 to fall in between these two. 



According to Forster (1916b), the ventral interossei of the spider 

 monkey are double, one arising at each side of every metacarpal 

 base and shaft I to V to end, respectively, on the ulnar and radial 

 aspects of the corresponding metacarpophalangeal articular capsules. 

 The dorsal interossei, on the other hand, are said to have the common 

 bicipital arrangement with digit III as the central axis. Jouffroy and 

 Lessertisseur (1959) describe five dorsal interossei in Ateles: one with 

 two heads in the first intermetacarpal space; another two-headed 

 dorsal interossei in the second; two bicipital muscles in the third, 

 one going to the ulnar side of digit III, the other to the radial side 

 of IV; and, finally, a single-headed dorsal interosseous along the 

 ulnar side of IV. This disposition corresponds to their paraxonique 

 type and is illustrated in their figure 3. Straus (unpublished) speaks 

 of seven interossei in the right hand of one Ateles geojffroyi, four 

 bicipital dorsals and three ventrals, the last arising from a single 

 metacarpal. He has found the actual separation between the dorsals 

 and ventrals not always complete, the first ending mostly on the 

 corresponding phalanges with some fibers reaching the dorsal extension 

 expansion, whereas the second has the reverse kind of insertion. This 

 pattern is like that of Alouatta. The information about Brachyteles 

 does not lend itself easily to the kind of comparison I am interested 

 in making because Hill (1962), who is the only source for these muscles 

 in the woolly spider monkey, does not clearly explain the origins of 

 what he calls dorsal and volar interossei. Robertson (1944) was 

 apparently faced with an extremely complicated pattern of interosse- 

 ous muscles in Lagothrix. The situation is probably similar to that of 



