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. polit'us Newb., from the Cleveland Shale of Ohio, may be found in the 

 Journal of Geology, Vol. VII., 1899, p. 492. 

 Formatmi 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, qixadrate 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 /. biiu- 

 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 widtli. 

 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 margin, thus aftordiiig 

 firm support for the functional ones (c/. Text-fig. 3) . 

 The teeth of each series are closely wedged to- 

 gether and interlock with those of adjoining rows, 

 the whole forming a very compact mass. 

 The arrangement of teeth in Janassa is well illustrated in the figures given 



Fig. 3. 



Diagram showing arrange- 

 ment of upper and lower 

 dentition in Janassa bitu- 

 minosa Schloth. (Slightly 

 modified after the resto- 

 rations by Hancock and 

 Howse, and 0. Jaekel.) 



xf. 



