FOSSILS OF THE BURLINGTON GROUP. 477 



inches. Hight of largest interambulacral plates, 0.26 inch ; 

 breadth of do., 0.40 inch; thickness of do., 0.25 inch. 



The only specimen of this fine species yet known to us is mainly a 

 silicious cast of the interior. The connection of the plates, however, are 

 so distinctly defined by sharply raised lines, formed by the silicious 

 matter deposited in the sutures between all of the pieces before they 

 were dissolved, that the entire structure can be made out as well as if 

 the plates themselves had been preserved. A few of the plates, how- 

 ever, or rather casts of their external surface, remain so as to show the 

 surface granules as well as the thickness of the plates themselves. 



The apical disc seems to be very similar, as already stated, to that of 

 M'lonites multipora the arrangement and comparative sizes, as well as 

 form of the occular and genital plates, being much the same. In two 

 of the latter, five pores may be counted in each, while one other also 

 shows obscure indications of five pores and the other two had four each, 

 as near as can be made out from the little projecting points represent- 

 ing them in the cast. No satisfactory indications of pores, however, 

 are to be seen in the occular pieces. 



Although the ambulacral areas are not properly furrowed, as in 0. 

 Dame and Melonites multipora, they are slightly depressed below the 

 most convex central region of the iuterambulacral areas. The depres- 

 sion, however, also includes the two marginal rows of each interambulac- 

 ral series. There is likewise a faint, narrow, almost linear impression 

 on the internal cast, extending from the apical disc about half way 

 down the middle row of plates in each interambulacral field. 



This form can be at once distinguished from 0. Dana, the only other 

 known species of this type, by its proportionally much larger and less 

 numerous interambulacral plates, of which there are only five instead 

 eight or nine rows to each area. Its ambulacral areas are also propor- 

 tionally narrower, and, as already stated, differ in not being furrowed 

 along each side, with a ridge along the middle. 



As we have elsewhere suggested, the group Oligoporus seems to be 

 exactly intermediate in its characters between 31'elonites, Owen and 

 Norwood, and Palcechinus, (Scouler) McCoy. That is, it differs from Pa- 

 IcEchinus in having four rows of ambulacral pieces and four double rows 

 of pores, instead of two of each, as well as in having the ambulacral areas 

 more or less sunken below the interambulacral fields. In the last char- 

 acter it agrees more nearly with Melonites, from which, however, it 

 differs widely in having only four rows of ambulacral pieces and four 

 double rows of pores, instead of ten of each to each area. In the nature 

 of its apical disc the species under consideration shows that in this type 

 it agrees well in its general characters with ^felonitex. We also know. 



