NOTES ON CARBONIFEROUS COCHLIODONTS 21 



The earliest mention of this supposed association is in a letter 

 written by Captain Jones to Portlock, and published in 1843, z i n 

 which he says : "I think I am enabled to show that, however distinct 

 and well marked the extremes may be, yet Helodus planus passes 

 into Cochliodus magnusP In connection with this statement no 

 evidence is given for its support. In publishing the results of his 

 investigations on the arrangement of Psephodus teeth, Davis says in 

 summary : 



That a row of three principal teeth increasing in size backward were attached 

 to each cartilaginous ramus of the jaw; that the diameter of the jaw, as indicated 

 by the groove or channel on the under surface of the teeth, diminished toward 

 the symphysis; that a long narrow tooth was placed in front of the anterior one; 

 and that a series of at least three helodoid teeth were placed behind it extending 

 over the palate and increasing in size backward. 2 



The same year St. John and Worthen published the results of 

 their studies on this genus and described an entirely different arrange- 

 ment of the teeth. They conclude that — 



While the median portion of the rami of the jaw of Psephodus was enveloped 

 by a moderately contorted dental plate, constituting its chief point of resemblance 

 to Cochliodus, this plate was flanked on either side by a series of teeth disposed 

 in rows from within outward similar to the occurrence of the teeth in the jaws of 

 Cestracion. Therefore the solid triturating plates of Psephodus are not strictly 

 homologous with the large posterior teeth of Cochliodus, but they are more properly 

 designated as median teeth of the rami of the jaws 3 



In 1885 Traquair published his conclusions as to the arrangement 

 of Psephodus teeth, saying: 



On the whole, I consider that view most likely to be correct which would 

 ascribe to the mouth of Psephodus four large tooth plates, two above and two 

 below, each occupying on the ramus a position similar to that of the row of the 

 largest teeth of Cestracion, or of the so-called median teeth of Cochliodus, but 

 that of the upper jaw differing slightly in form from the one opposed to it below. 

 .... Teeth of different forms seem to have been present in the jaw, external to 

 and in front of the large plate; those in its immediate vicinity belonging more or 

 less to the category of Helodus planus But the front of the jaw was armed 



1 Portlock, Geological Report on Londonderry (1843), p. 462. 



2 Scientific Transactions of the Royal Dublin Society, Vol. I (1883), p. 417, Plate 

 LV, Figs. 1-4. 



3 St. John and Worthen, Paleontology 0} Illinois, Vol. VII (1883), p. 64. 



