FISH OF THE CARBONIFEROUS PERIOD. 5 



ternal conformation) by the much greater coarseness of the 

 calcigerous tubes, and the greater space of blastema separa- 

 ting them : the difference is still greater towards the exterior ; 

 for while, in Rhizodus, the coarse tubes of the dentine terminate 

 abruptly near the surface, ending in a layer of minute calci- 

 gerous cells, from which the infinitely finer and closer, straight 

 tubes forming the enamel-like surface take their origin ; in Co- 

 lonodus the loosely flexuous calcigerous tubes are four or five 

 times their diameter apart, and as they approach the surface they 

 gradually become finer, a little straighter, closer and more nu- 

 merous ; but there is no layer of cells, no abrupt line of separa- 

 tion between the coarse tubes of the body of the tooth and the 

 fine ones of the surface. The simplicity of its microscopic struc- 

 ture, and the simple round base distinguish Colonodus from cen- 

 tral cusps of Cladodus "(Ag.) . 



Colonodus longideMs (M^Coy). 



As there is but one species yet known, it is not possible to 

 separate clearly the specific from the generic characters. How- 

 ever, the specimen on which those observations have been made 

 is an almost perfectly straight, cylindrical tooth, the apex being 

 unfortunately wanting, but enough remains to show that towards 

 the extremity the anterior face becomes flattened so as to give 

 an obscurely trigonal section ; there are two alternating rows on 

 each side of about thirteen or fourteen short transverse furrows, 

 forming between them obscure wrinkles ; the whole surface to 

 the naked eye seems smooth and highly polished, but under a 

 low power the fine, impressed, rather distant longitudinal sulci 

 become visible. The whole tooth seems directed backwards at a 

 considerable angle from its round bony base, and the inferior 

 termination of the enamel-like portion is therefore very oblique 

 to the axis of the tooth, being considerably lower in front than 

 behind, the edge seeming of considerable thickness from a sharp 

 constriction being immediately under it all round, beneath which 

 again the osseous base thickens to form a little peltate mass. The 

 length of the portion preserved is 10 lines, width at base 3 lines, 

 broken extremity at the above length IJ line. 



From the red carboniferous limestone of Armagh. 



[Col Capt. Jones, R.N., M.P.) 



Placodermi. 



I provisionally propose to establish a distinct family under the 

 above name, to include those Ganoid fish of the palaeozoic rocks 

 having the head and body encased in a series of odd or central, and 

 of subsymmetrical or lateral, bony, variously tuberculated plates 



