Collette and Gillis: Osteological differences between two species of Grammatorcynus 



37 



1975, Gibbs and Collette 1967). 

 In Grammatorcynus the poster- 

 ior-most neural and haemal spine 

 bend away from the vertebral 

 axis and parallel the dorsal and 

 ventral edges of the hypural plate. 

 In Acanthocybium and Scombero- 

 monts, three posterior neural and 

 haemal spines bend away from 

 the vertebral axis more abrupt- 

 ly than in Grammatorcynus. 



The triangular hypural plate is composed of 5 fused 

 hypural bones (Potthoff 1975). In some specimens of 

 Grammatorcynus (G. bilineatus 453 and 521mm FL, 

 and G. bicarinatus 563mm FL) the dorsalmost (hypural 

 5) is partially fused with the dorsal part of the hypural 

 plate (hypurals 3-4). However, in smaller specimens 

 (382-424 mm FL) such fusion was absent, as is the case 

 in Scomberomorus and Acanthocybium. There is a 

 primitive hypural notch present on the middle of the 

 posterior margin of the hypural plate. This notch is a 

 remnant of the fusion of the dorsal part of the hypural 

 plate with the ventral part (hypurals 1-2). The notch 

 is absent in the more advanced bonitos and tunas 

 (Collette and Chao 1975). 



The parhypural is separate from the ventral hypural 

 plate in Grammatorcynus and Scomberomorus but is 

 fused with it in Acanthocybium. This fusion was also 

 noted by Conrad (1938), Fierstine and Walters (1968), 

 and Devaraj (1977). The two haemal arches preceding 

 the parhypural are autogenous in the three genera, 

 although Devaraj (1977) stated that they were fused 

 with their centra in Acanthocybium.. 



The parhypural has a strongly-hooked process, the 

 parhypurapophysis (or hypurapophysis), at its proximal 

 end. The parhypurapophysis slopes slightly upwards 

 similarly in Grammatorcynus and Scomberomorus. In 

 Acanthocybium it has a right angle and then a level 

 projection. 



There are two epurals as in other scombrids (Potthoff 

 1975). In shape and size, the anterior epural (1) resem- 

 bles the neural spine of adjacent preural centrum 3. The 

 smaller, posterior epural (2) is a free splint located 

 between the anterior epural and the uroneural and fifth 

 hypural, which are joined together. 



Dorsal and anal fins Grammatorcynus usually has 

 12 dorsal spines, rarely 11 or 13 (Table 4), fewer than 

 either Scomberomorus (12-22) or Acanthocybium (23- 

 27). Dorsal spine counts are roughly correlated with 

 vertebral number: Grammatorcynus has the fewest 

 precaudal, caudal, and total vertebrae, and the fewest 

 dorsal spines, while Acanthocybium. has the most 

 precaudal and total vertebrae, and the most dorsal 

 spines. 



The range in number of second dorsal fin rays is 

 10-25 in the three genera. Grammatorcynus has 10-12 

 rays, 10 in G. bicarinatus and usually 11 in G. biline- 

 atus (Table 4). There are usually more second dorsal 

 rays in Acanthocybium (11-16) and Scomberomorus 

 (15-25). 



Dorsal finlets number 6-11 in the three genera. 

 Grammatorcynus has 6-8, usually 7 in G. bicarinatus, 

 and usually 6 in G. bilineatus (Table 4). Acanthocybium 

 has 7-10, and Scomberomomis has 6-11. The total 

 number of second dorsal elements is the same in both 

 species of Grammatorcynus, 11-1-6 = 17 in G. biline- 

 atus, 10-1-7 = 17 in G. bicarinatus. 



Anal fin rays show a similar trend to that of dorsal 

 fin rays. The range in the three genera is 11-29. Gram- 

 matorcynus has 11-13 (Table 5), similar to Acantho- 

 cybium (11-14), but much fewer than Scomberomorus 

 (15-29). 



Anal finlets range in number from 5 to 12 in the three 

 genera. Grammatorcynus has 5-7, usually 6 in 

 G. bilin£atus, and usually 7 in G. bicarinatus (Table 5), 

 generally fewer than Acanthocybium (7-10) or Scom- 

 beromorus (5-12). Again, the total number of anal 

 elements is the same in both species, 12-1-6 = 18 in 

 G. bilineatus, ll-i-7 = 18 in G. bicarinatus. 



Pectoral girdle The pectoral girdle consists of the 

 girdle itself (cleithrum, coracoid, and scapula), the 

 radials to which the pectoral fin rays attach, and a chain 

 of bones that connect the girdle to the rear of the skull 

 (posttemporal, supracleithrum, supratemporal, and two 

 postcleithra). 



