54 Messrs. Hancock & Howse 07i Janassa bituminosa 



pliysis ; on each side of this central tooth are a first and a se- 

 cond asymmetrical 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 petalodon- 

 toid 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 

 horizontal rows, the teeth diminishing in size downwards, 

 the lower ones having been first developed, and in succes- 

 sion 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 ; Avhile at the 

 same time the dentigerous membrane is pushed forward, and 

 the oldest row, the lowest in the series, or that Avliich was first 

 develo^^ed, falls away. Thus, by this double action of growth 

 and decay perpetually going on, there is always an efficient 

 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 

 extraordinary, as it is common to all the Sharks and Rays, 

 the close allies of Janassa. 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 dentition, with the exception of the so-called (7//w«a.r- 

 odus lingua'formis ; and that interesting Coal-measure species 

 has been shown to be a true Janassa, in the paper previously 

 quoted from the November number of the ' Annals.' The 

 only instance that occurs to us in which something similar is 

 found, is seen in the Greenland Shark, SqnaJus borealis, 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 ofPetalodus, 

 we believe, have also been found lying in regular order, as if 

 forming a portion of a vertical row. 



This curious pile of teeth forms a close, dense mass, increasing 

 in size upwards, or as the last-developed teeth are approached — 

 the smaller rows of teeth, as already stated, being below; and 

 the teeth themselves are, as it were, interlocked. The central 

 teeth of each horizontal row are the only ones that are placed 

 exactly above each other ; the lateral teeth of the successive 

 rows are arranged in quincunx; so that they may be looked 



