Infraoral lamina were studied of 630 lampreys from Lake Erie, Ontario, Lake Seneca, New York, and the Père Marquette River, Michigan. The number of cusps on the first set of the lamina is always repeated in consecutive new sets, but the size of the cusps and their orientation vary. The width of the lamina increases slowly from 4.5 to 16.5 mm, with successive replacements of the corneous sheath. The main types of lamina are: normal (teeth large, evenly spaced, their points nearly parallel), inclined (teeth large, evenly spaced, their tips inclined mesad), and rosebud (teeth very small and clustered near the middle of the lamina; concentric lines on the basal portion). There is also the type intermediate between inclined and normal. Newly transformed lampreys and others less than 180 mm long have normal laminae. The rosebud lamina is characteristic of half-grown specimens, being most frequent among lampreys 250–400 mm long. In adults, feeding in lakes, the predominant type is the inclined lamina. In spawning specimens the normal lamina is most characteristic. Thus the usual succession throughout life is: normal, rosebud, inclined, and finally normal again. In spawning specimens there are two new sheaths underneath the old one, one below the other.
Offshore sediments in the Lake Erie Basin, Ontario, Canada, and Michigan, Ohio, Pennsylvania, and New York, U.S.A.
