506 PROCEEDINGS OF THE NATIONAL MUSEUM vol. 83 



irregularities of surface, and are tied together by connective tissue and 

 the sldn and thus form a segment. The plates of each segment also 

 overlap with those of the adjacent segments to form one continuous 

 ridged exoskeleton. A typical tail segment is thus quadrangular. A 

 typical trunk segment, in addition to the four plates just described, 

 has an irregularly oblong or elongate plate interpolated on the side 

 between the dorsolateral and ventrolateral plates, and another, 

 unpaired, rouglil}^ trough-shaped plate is interpolated between the 

 two ventrolateral plates. The midventral plate is absent on the last 

 trunk segment. The longituduial ridges on the series of interpolated 

 plates on the side form the lateral ridge of the trunk, while the scries 

 of troughlike plates forms the midventral ridge. The trunk thus 

 typically has three ridges on each side and one midventral ridge, the 

 latter extending only to the penultimate segment. A typical trunk 

 segment is thus septangular. 



The fact that the midventral ridge does not extend to the last 

 trunk segment causes that segment to assume a different shape from 

 the preceding one, but the chief cause of the modifications of the seg- 

 ments in the region where the tail and trunk meet is the structure of 

 the base of the dorsal fin, which is on a more or less raised platform. 

 The elevation is caused partly by one or more extra plates present 

 there, forming a support for the dorsal. The number of extra plates 

 and their positions differ with the species. The presence of these 

 plates results in certain changes in the external appearance of that 

 region, which are next described. 



As already noted, a typical caudal segment is quadrangular. In 

 most species the first caudal segment normally has an extra plate 

 and is thus hexangular, with few individual variants; in one species, 

 regulus, the first caudal segment nearly always lacks the extra plate 

 and is normally quadrangular like the following segments; while in 

 another species, 2osterae, it is usually quadrangular but often hex- 

 angular, the frequency of the hexangulate condition depending on 

 the local population of that species. 



The last trunk segment nearly always bears an extra plate on top 

 for the support of the dorsal in the species described here, and this, 

 together with the lack of tlie midventral ridge, causes it to be normally 

 octangular, with one exception, zosterae, in which it often lacks the 

 extra, plate and is then hexangular. The penultimate trunk segment 

 usually lacks an extra plate, but it has a midventral ridge and is 

 consequently normally septangular, like the segments preceding it. 

 Sometimes, however, it has an extra plate and is then novemangular. 

 The novemangular penultimate trunk segment is present as an indi- 

 vidual variation with greater or lesser frequency in nearly all the 

 species examined, becomes nearly the dominant condition in ivgens, 

 and is normal in the subgenus Alacleayina. 



