RICHARDS. MILLER, and HOUDE: DEVELOPMENT OF THREAD HERRING 



Table 3. — Position of dorsal and anal fins during the transformation period in 

 larvae of Opisthonema oglinum. 



joined at the anterior tip of the upper jaw, and 

 the maxillaries appeared to be fully formed. 



At about 10 mm, the cleithral ring was obvious 

 as a thin ring of ossified bone almost encircling 

 the head and forming a posterior line of de- 

 markation. The three bones that actually form 

 bhis ring are the supracleithrum, cleithrum, 

 and postcleithrum, which developed fairly uni- 

 formly to about the time the pectoral fin began 

 to develop ossified rays (about 15 to 17 mm). 

 Thereafter, the cleithrum became the dominant 

 element. 



A cartilaginous cranial bulb was visible in 

 our smallest larvae, but no elements were dif- 

 ferentiated or ossified until about 12 mm. At 

 that size, bone elements (e.g., the sphenotics, 

 parietals, epiotics, and pterotics) were slightly 

 outlined. Between 12 and 13 mm, the vomer and 

 parasphenoid began to show considerable ossi- 

 fication, and the pterotic and prootic bullae were 

 differentiated by ossification of the surrounding 

 bones. The frontals began to form when speci- 

 mens measured about 15 mm, and about 19 mm 

 they were partly ossified (but not fully formed), 

 and sensory canals were present within the bones. 

 Canals also formed at the 19-mm size in the 

 prootics and sphenotics, and the prootic bulla 

 was the most obvious structure in the skull be- 

 cause of the heavily ossified bone around it. 



The quadrate and the pterygoids had begun 

 to form by 19 mm, and the articular had in- 

 creased in size but was not yet fully ossified. 

 At about 21 mm, the nasals began to form as 



thin, small plates at the anterior ends of the 

 frontals. Formation of supraorbitals had also 

 begun, the anterior one being moderately well 

 developed and quite separate from the posterior 

 one. At the 21-mm size, it also appeared that 

 most of the head bones were at least partly 

 formed, and ossification ranged from slight to 

 considerable. By 25 mm, the prootic bulla was 

 well encased in heavy bone, the sclerotics had 

 begun to ossify along with the supraorbitals, 

 the ethmoids had begun to form and ossify at 

 the anterior ends of the frontals and just behind 

 and above the premaxillaries, the vomer was 

 quite large and well ossified, most sensory canals 

 were partly to fully formed and encased in bone, 

 the postorbitals had begun to form, and the 

 pterotic bulla (which lagged far behind the pro- 

 otic in development) began to grow larger. From 

 25 mm through the juvenile period, growth of 

 the skull bones was restricted primarily to an 

 increase in size and consolidation within the 

 several multibone complexes; the dorsal and 

 lateral frontoparietal foramina decreased in size 

 as the bones increased, and the characteristic 

 striae of the dorsal surface of the frontoparietals 

 began to form; the pterotic bulla finally became 

 larger than the prootic by about 34 mm; and 

 the palatines began to ossify. During the 30- to 

 50-mm range, most of the cranial structures 

 assumed their adult configuration, the dorsal 

 frontoparietal foramina completely closed up, 

 and the lateral foramina became reduced in rela- 

 tive size. 



1133 



