37O T. D. A. COCKERELL. 



in the scale of fish-evolution, these radiating lines or radii 

 often become very prominent, while the longitudinal strands 

 usually become united above and below, forming circular 

 fibers which we have designated circuli. The nomenclature of 

 these structures was based on a normal highly-developed scale, 

 in which the circuli deserved their name, and since then the 

 term has been applied to the same elements wherever found, 

 so that I have had to refer, rather illogically, to longitudinal 

 circuli. Perhaps it would be better to call them fibrillae. 



On looking at a normal scale, in which the circuli are strictly 

 circular, it would be natural to regard them as lines of growth, 

 like those on a snail's shell. There are real lines of growth, how- 

 ever, and these do not necessarily coincide with or have anything 

 to do with the circuli. 



In the salmon and trout (Salmo) the scales are strictly cycloid, 

 and have only circuli. Fig. 2 shows a scale of the bluefin, 

 Argyrosomus nigripinnis, a member or the salmon family. This 

 show r s a marked deviation from the simple Salmo type, in that 

 there are distinct laterobasal angles, the circuli of the apical 

 field are less dense than those of the basal, and there are slight 

 indications of apical radii. It is no very great step from this to 

 the scale of Moxostoma aureolum, one of the suckers (Fig. 3), 

 but it will be noted that in the sucker, as indeed in all members 

 of the Catostomidae so far examined, there are very distinct 

 basal radii. In the typical suckers, Catostomus, the scale is oval, 

 not unlike that of Salmo in shape, and there are radii all around. 

 This is well shown in Fig. 4, Pantosteus santa-ance, from Califor- 

 nia. The possession of basal radii separates the Catostomidae 

 from the great majority of American cyprinids, although a few 

 genera of the latter family have them, notably Chrosomus (Fig. 5, 

 C. dakotensis], the scale of which closely resembles that of 

 Pantosteus in sculpture, though differing in shape. Among the 

 old world cyprinids (carp family) basal radii are very common, 

 and it is curious that the common European minnow (Phoxinus 

 phoxinus) has scales of quite the same type as the American 

 Chrosomus. There is reason to believe that the suckers are an 

 ancient family, close to the old stem from which the carp family 

 arose. They are nearly confined to North America, though spar- 



