SCALE OBSERVATIONS OF THE SQUETEAGUE AND PIGFISH. 295 



RADII. 



The first important hypothesis dealing with the nature of the radii was that of 

 Agassiz (1840), who thought them "channels at the margin of the external surface 

 which connect one layer with another and multiply during the growth of the scale." 

 Mandl (1840) considered them as canals for transporting nutrition to the center of the 

 scale. Peters, instead of giving them the function of connective canals, regarded them 

 as sutures allowing growth in all directions. He also notes that they are sometimes 

 concentric, as in Ophidium. 



Williamson (1849) denied the existence of any such canals as Mandl described. 

 He says that they are simply the absence of the superior tissue along their course. 

 While they are not nutrient canals, neither do they pass through the entire calcareous 

 portion of the scale and reach the soft portion, as Agassiz contended. 



Salbey (1868) says that the radii are grooves in the superficial layer, but not through 

 what he calls the conjunctive layer, and suggests that they are the channels for the 

 continued calcification of the interior conjunctive substances which calcify slowly and 

 are not in juxtaposition to any other nourishing parts. Tims considers the radii as 

 adaptations to the increasing circumference. 



Baudelot (1873) thoroughly describes radii, both as to structure and disposition. 

 Aside from numerous variations, all of which he records minutely, mention might be 

 made of the three main modes of disposition. These are (i) divergent from the focus; 

 (2) parallel with each other; and (3) parallel with the contour of the scale. Their 

 form may be that of simple lines of fissures in which the scale appears to be broken; a 

 ravine whose sides are perpendicular with the sides of the scale; a wide and shallow 

 trench; a groove of varying width; a series of depressions, or, in some cases, a series 

 of small cavities in the same straight line. In regard to number, he says, "The number 

 of radii of an indi\'idual is capable of varying with age, and if the number increases 

 with age it may also be reduced." The same conclusions apply to the transverse or 

 concentric grooves. 



Baudelot pointed out that up to his time no satisfactory explanation of the radii 

 had been offered, and in his attempt to explain them he attributes them to irregular 

 calcification. He says : 



Grooves arc lines of noncalcification. The exterior layer has centers of calcification which later 

 unite with one another as these centers extend. V/hen the union takes place laterally the grooves will 

 be radii ; otherwise they will be transverse grooves, and when calcification takes place all over at the 

 same time there will be no radii. 



Cockerell (1911) finds radii on both the anterior and posterior fields, calling the 

 former "basal" and the latter "apical" radii. He attaches enough importance to 

 their number to make it a taxonomic character. 



FOCUS. 



The center of the scale would, to most observers, suggest a center of growth; never- 

 theless, it has been the subject of much conjecture. Vogt (1842) first noticed that the 

 focus is larger in the adult than in the young, suggesting wear, unless the scale, in, its 

 entirety, increases in size — a supposition very difficult to substantiate. The com- 

 paratively large focus in some cases suggested to Agassiz (1834) that it might be the 

 result of wearing down of the thickened center. Both Peters (1841) and Salbey (1868) 



