THE STURGEONS AND STURGEON INDUSTRIES. 
241 
rhombic scutes, just as in all probability the dorsal, lateral, and ventral rows of scutes 
were in part formed, but at a very much earlier stage. 
The decreased roughness of the skin as observed in the adult of A. sturio is due in 
part to the erosion of the apices of the dermal denticles and the loss of the acuminate 
tips of the larger scutes, and in part also to the fact that the integument gradually 
thickens and the basal ossifications sink into it more deeply. This is well illustrated 
by the history of the preanal plates. These plates in the young are very obvious 
externally as a closely aggregated group, but in the adult they frequently become so 
deeply embedded in the integument that one must feel for their presence through the 
abdominal walls of this region. 
The gradual loss of integumentary asperities is apparent in other regions in the 
transition from youth to age. This is especially noteworthy in regard to the pair of 
large, nearly rhombic bucklers between the bases of the pectorals just behind the 
isthmus. In the young of A. sturio these bucklers have strong cariuae along their 
inner, longitudinal borders ; in the adult, on the contrary, these carinm are quite oblit- 
erated externally. 
The young sturgeon, as a consequence, is provided with a dermal armor which is in 
some respects much more efficient than that of the adult. One is reminded very 
strongly of the strong, sharp spines which are found on the heads and edges of the 
opercles of the young of many strictly marine fishes, where in some forms, such spines 
are so strongly developed as to render it difficnlt to realize that they are eventually 
suppressed so as to be practically without any spinous defensive apparatus in the 
adult stage. 
That such a change occurs in the sturgeons generally is proved by the very promi- 
nent retrorse spines found surmounting the ijosterior moiety of each of the dorsal, 
lateral, and ventral plates of the young from 9 to 18 inches in length. 
In still younger stages of A. huso under 2 inches in length, according to the figures 
of Brandt and Ratzebnrg, Plate XLIV, Fig. 22, this central, thorn-like portion of each 
of the bucklers is still more prominent than in specimens a few inches longer. As the 
animal grows still larger, as in examples ot from 6 to 10 feet in length, the retrorse, 
thorn like character of the median prominence on each buckler vanishes, and all that 
remains to indicate its former presence is a very low conical, or ridge-like elevation 
in the middle of each plate. This change in the external armature of the bony plates 
is due partly to the manner in which they increase in size and partly to the erosion of 
the prominent external median portion. 
The advantage of the markedly rougher armature of the young in the struggle for 
existence is obvious, as it is clearly adapted to render the young animals less conven- 
ient of deglutition or mastication by the more ravenous predaceous forms inhabit- 
ing the same waters. 
The marked difference in external features between the young and the adult has 
invited the misguided attention of systematic writers, chief amongst whom must be 
mentioned Auguste Dum6ril, who has divided the genera found in various parts of the 
world upon the basis of the position of the armature of the bony integumentary plates. 
As a little acquaintance with the development of these fishes would enable any one 
to predict, the spinous prominences are found in a posterior position in the small 
species, while in the larger species this prominence is about the center of each plate. 
Tlie groups, Opisthocentres and Mesocentres, of Dumeril are therefore founded upon more 
Bull. U. S. F. 0., 88 16 
