The minute and curious marks so produced are known as haloes; for they surround, in ringlike forms, minute particles of included substances which contain radioactive elements.
"The Birth-Time of the World and Other Scientific Essays"
J. (John) Joly
The lowest in grade and nearest, perhaps, to the ancestral type of vertebrates, was the problematic creature, an inch or so long, of Figure 297. Note the circular mouth not supplied with jaws, the lack of paired fins, and the symmetric tail fin, with the column of cartilaginous, ringlike vertebrae running through it to the end.
"The Elements of Geology"
William Harmon Norton