ON JANASSA BITUMINOSA. 847 



the longitudinal median line, or exactly over the symphysis ; on 

 each side of this central tooth are a first and a second asymme- 

 trical primary tooth, making up the five primaries. These are 

 flanked on either hand with a single secondary or petalodontoid 

 tooth, completing the full complement of seven. They diminish 

 in size from the centre, the flanking petalodontoid teeth being 

 quite small in comparison with the large central primary tooth. 



The rows are placed one above the other in horizontal ranges, 

 the lower rows acting merely as mechanical supports to the 

 upper row, or that which was alone employed in cutting and 

 crushing the food. There are from four to seven such horizon- 

 tal rows, the teeth diminishing in size downwards, the lower 

 ones having been first developed, and in succession having had 

 their period of active operation. As they wear out (that is, as 

 the cutting-margins become blunt, and as the imbricated ridges 

 of the cutting-disks are obliterated or reduced), a new row is 

 developed behind, and, rising up, falls forward, and rests upon 

 the row last in use ; while at the same time the dentigerous 

 membrane is pushed forward, and the oldest row, the lowest in 

 the series, or that which was first developed, falls away. Thus, 

 by this double action of growth and decay perpetually going on, 

 there is always an efl&cient row at the surface, able to initiate 

 the process of alimentation, sustained at a proper elevation on a 

 firm basis. 



This constant renewal of the oral armature is nothing extra- 

 ordinary, as it is common to all the Sharks and Eays, the close 

 allies of Jcmassa. But that the new set of teeth should overlie 

 and be supported by the old ones is indeed without a parallel, 

 so far as we are acquainted with the subject of ichthyic denti- 

 tion, with the exception of the so-called Climaxodus livgucpformis ; 

 and that interesting Coal-Measure species has been shown to be 

 a true Janassa, in the paper previously quoted from the Novem- 

 ber number of the "Annals." The only instance that occurs to 

 us in which something similar is found is seen in the Greenland 

 Shark, Squahis boreaUs, in which the older teeth of the lower 

 jaw lie in front of, and give support to, the last developed or 

 those in use. Teeth of Petalodus, we believe, have also been 



