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49 



towards the ventral margin and the ribbed arrangement of the sculpture more 

 open and irregular. 



The bones of the cranium are also sculptured, like those of the dorsal cuirass. 



The ventral cuirass consists of a row of 14 (sometimes 13 or 15) large, 

 regular plates, the lowermost part of which forms the above-mentioned, thin, 

 transparent edge. The anterior ventral scutum is in part covered by the preoper- 

 culuni. In A. scutata the scuta 4—14 have a narrow, thickened, more solid stripe 

 or rachis, / fig. 1 PI. I, which arises at the upper margin of the transparent keel 

 and forms above a prominent fiat projection on the upper edge of the scale; the 

 stripes on the 5th and 6th scuta are placed obliquely, diverging above, on the 

 other plates they are vertical ; on the three posterior scuta they do not reach the 

 margin of the dorsal cuirass. No. 5 is the largest of all the plates; between its 

 upper, anterior corner and the clavicula we find a small, separate plate (s), which 

 at first glance seems to be a somewhat widened upper part of the rachis on the 

 5th ventral plate. No. 1 is of fairly good size, longer than the two following; 2, 

 3, 13 and 14 (15) are the smallest. 



The ventral fins are inserted into the 8th plate. This holds good also for the 

 other species; in these likewise the ventral plates 5—14 are provided with a rachis 

 as in scutata; but no. 4 lacks this completely or has only a slight indication of it; 

 the 5fh plate is not, or not appreciably, larger than the 6th and has thus nothing 

 more remarkable about it in comparison with the others than the oblique, posteri- 

 orly inclined rachis (cf. fig. 2, 3); the upper end of the latter in these species is 

 also continued into a rachis for a small, separate plate (s), which forms the con- 

 nection with the clavicle, but this plate is here considerably larger than in scutata. 

 The anterior plate no. 1 is shorter than in the latter species, not or but little longer 

 than 2 and 3, and is thus covered as a rule completely or almost completely by 

 the preoperculum; nos. 2, 3, 12 — 14 are the smallest; also in these species the 

 rachides of the posterior plates do not reach to the dorsal cuirass. 



When the number of the ventral plates is increased to 15, a plate is added 

 posteriorly in front of the anal plate; when the number is reduced to 13, the two 

 penultimate plates are fused together. 



It is difficult to determine with certainty whether the 14 ventral plates are 

 paired or unpaired formations — recalling in the latter case the ventral scales of 

 the herring. The 8th plate can obviously be divided into a right and a ventral 

 part, at least from the place where the ventral fins are fixed and the pelvic region 

 protrudes on the ventral margin ; we also find in most of the specimens of all 3 

 species, that the posterior, 14th, plate is paired, as its two sides tend to separate 

 from one another (without preparation) and thus the sharp, ventral margin appears 

 divided. But all the others appear in fact unpaired, the two sides of each plate 

 being united to the thin ventral keel; it has at any rate been impossible for me 

 to separate this into two lateral halves by maceration or by the use of potash. 

 The marginal part of the transparent keel consists of a very hard, dense and firm, 



7* 



