one unpaired: the ventral plate (scutum abdominale) and 3 

 paired, the superior, median and inferior lateral plates (scuta 

 lateralia superiora, media et inferiora); in most genera there 

 are small intermedial scutes (scutella) between the lateral scutes. 

 Only the first or scapular ring of DUNCKER is more compli- 

 cated. The first ring consists of the "Skapularschild" of 

 DUNCKER = "dermal part of the clavicle and the coverplate" 

 of JUNGERSEN and DUNCKER'S " Axillarschild" = JUNGERSEN'S 

 median lateral plate of the second ring and in the third place 

 of JUNGERSEN'S "jugular plate" = "Infrascapulare" of DUNCKER. 

 The praenuchal plate (shield), between the occipital crest of 

 the skull and the first spinous process and the nuchal plate 

 (shield) between the spinous process of the first and second 

 vertebrae are possibly modified interspinous bones. 



The scapular ring of DUNCKER consists, according to JUN- 

 GERSEN, in reality of the elements of two rings. But as the 

 second ring, that behind the pectorals, is the first ring of typical 

 'composition, we follow DUNCKER and count both, for conve- 

 nience sake, as the first ring of the trunk (cfr. Fig. 16). 



The postdorsal or tail rings contain only the paired scuta 

 lateralia superiora et inferiora. 



The scuta of the same name are antero-posteriorly linked 

 together by a kind of articulation. Each scutum (shield) is 

 provided with a median longitudinal keel, and as the keels of 

 the succeeding scuta fit together, the succeeding keels produce 

 the longitudinal cristae of trunk and tail, the number of which 

 corresponds with the arrangement of scutes in trunk and tail. 

 We distinguish therefore on the trunk the unpaired crista abdo- 

 minalis trunci, which terminates on the last or forelast trunk- 

 ring, the paired cristae laterales superiores, mediae et inferiores 

 trunci and on the tail the cristae laterales superiores et inferiores 

 caudae. In the subdorsal region, where generally the last 

 trunk-rings and the foremost tail-rings carry the dorsal fin, the 

 paired keels of trunk and tail are continuous or they are in 

 different way discontinuous. There exist, according to DUNCKER, 

 the following modifications: 



I. Superior cristae of trunk and tail continuous. 



1. Inferior cristae of trunk and tail discontinuous. Median 



cristae of trunk and inferior cristae of tail continuous * . * fig. 17, n. 1, 



2. Inferior cristae of trunk and tail continuous. 



