RICHARDS, MILLER, and HOUDE: DEVELOPMENT OF THREAD HERRING 



Table 2. — Measurements of larvae and juveniles oi Opisthonema oglinum. Specimens between dashed lines are undergoing 



notochord flexion. 



structures were clear, however, indicating no 

 ossification. The first structures to ossify were 

 the dentaries and maxillaries when specimens 

 measured about 10.0 mm (bone nomenclature 

 follows Mead and Bradbury, 1963). Almost simul- 

 taneously, the very thin cleithral ring (cleithrum, 

 supra-cleithrum, and postcleithrum) and the 

 hypurals began to ossify very slightly. At sizes 

 between 12 and 13 mm, the skull bones began 

 to ossify, as did the vertebral rings. As size 

 increased, various other parts of the fish began 

 to ossify — the caudal fin rays, parts of the 

 branchial apparatus (ceratobranchials, hypo- 

 branchials, epibranchials), and the dorsal and 

 anal fin rays; skull bones ossified further (Figure 

 3). At about 20 to 22 mm, essentially all bony 



structures of the larvae had at least begun to 

 ossify, and some were well developed. 



Vertebral development 



No ossification took place before 10 mm, but 

 cartilaginous structures were visible. At the time 

 of notochord flexion, seven hypural elements 

 were formed (four superior and three inferior 

 elements including the parhypural). In specimens 

 measuring 10.5 mm, the hypurals were weakly 

 ossified but the seven elements were distinct. 

 The first vertebral centra to ossify was the first 

 preural centrum and the ventral portion of the 

 centra of the first and second ural centra. This 

 differs somewhat from the development noted in 



1127 



