54 Messrs. Hancock & Howse on Janassa bituminosa 



physis ; on encli side of this central tooth arc a tirst and a se- 

 cond asynnnetrioal jirimavy tooth, niai<in<j^ uj) tlie five |n'iniaries. 

 These are tianked on either hand with a sin_ii;le seconthiry or 

 i)etahxlontoid tooth, completin^i; the full com])lemcnt of seven, 

 riiey diminish in size from the centre, the Hankinp^ petalodon- 

 toid teeth being (juite small in comparison with tiic large 

 central primary tooth. 



The rows arc j)laeed one above the other in horizontal 

 ranges, the lower rows acting merely as mechanical suj)])orts 

 to the up])er 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 tirst 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, tails 

 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 etficient 

 row at the surface, able to initiate the })rocess of alimentation, 

 sustained at a proper elevation on a firm basis. 



This constant renewal of the oral an^/.ature is nothing 

 exti'aordinary, 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 iehthyic dentition, Avith the exce])tion of the so-called CV//»«^r- 

 odiis Jinguaformis ; 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, Squalus borealis, in 

 which the older teeth of the lower jaw lie in front of and give 

 supjiort to the last-developed or those in use. Teeth oi'Peta/odus, 

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

 forming a portion of a vertical row. 



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

 in size ui)wards, or as the last-developed teeth arc 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 



