. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 170 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. is represented a little too conspicuously. In Figure 39 of the same plate is shown a tooth belonging to another species of Phoebodus. A good figure of P. politus Newb., from the Cleveland Shale of Ohio, may be found in the Journal of G-eology, Vol. VII., 1899, p. 492. Formation and Locality. — Permo-Carboniferous; Blue Springs, Nebraska. PETALODONTIDAE. Fortunate discoveries of Janassa afford the means for a clear understanding of the dentition and form of body in
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology; Zoology. 170 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. is represented a little too conspicuously. In Figure 39 of the same plate is shown a tooth belonging to another species of Phoebodus. A good figure of P. politus Newb., from the Cleveland Shale of Ohio, may be found in the Journal of G-eology, Vol. VII., 1899, p. 492. Formation and Locality. — Permo-Carboniferous; Blue Springs, Nebraska. PETALODONTIDAE. Fortunate discoveries of Janassa afford the means for a clear understanding of the dentition and form of body in the ray-like creatures belonging to this family. Janassa exhibits a ray-shaped trunk cov- ered with smooth, rounded, quadrate granules, and large pectoral fins which extend forward to the head, the pelvic pair being separated from them by an interspace. There are no fin-spines, the mouth- cleft is very narrow, as in rays, and the tail is slender. There can be no doubt that forms like this, or like Tamiobatis, Copodus, Psammodus, Archaeobatis, etc., were early approximations to the modern ray type, whether we consider them as genetically related to the latter or not. The dentition of Janassa, as determined with entire accuracy by Hancock and Howse in /. bitu- minosa, is similar in both jaws, and consists of a median or symphysial, and three pairs of lateral series, each having from four to seven teeth, the lateral series diminishing regularly in size from the center outwards. The lower dentition is more strongly arched and at the same time less extended from side to side than the upper, and the cutting- margins of the lower functional teeth bite inside those of the opposite jaw. The teeth of the outer- most lateral series in the upper jaw slightly exceed those of the corresponding lower rows in width. The manner of succession is peculiar in that the oldest-formed teeth, after they have ceased to be functional, become piled upon one another in front of and away from the oral m
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