228 



PALAEONTOLOGY OF ILLINOIS. 



the ambulacral and interambulacral pieces with the genital and ocular plates. 

 It also represents pores in each of the ocular pieces, and only three in four of 

 the genital pieces, and two in the fifth; while it represents the genital and ocu- 

 lar plates of nearly equal length. As may be seen by the annexed figures, the 

 specimens examined by us have the ocular pieces much smaller and shorter 

 than the others, and entirely without any traces of pores,* while one of the 

 specimens before us shows distinctly four pores in three of the genital plates, 



Fig. 21. 



Apical disc of Melonites multipora : Showinj 



and five in each Fig, 22. 



of the other two. 

 Another specimen 

 (annexed fig. 22) 

 shows five pores in 

 three of the geni- 

 tal pieces, and four 

 in each one of the 

 others. In order 

 to be the more sure 



that we were not mistaken in regard to the 

 number and arrangement of the pores, we 

 polished the surface of the whole disc in 

 both of these specimens with fine emery, 



the anal opening surrounded by the genital an d owing to the white Opaque character 

 and ocular pore3, and the connection of the ° r i 



latter with the ambulacral and interambulac- f th e plates, and the fact that the pores 

 ral pieces. All enlarged about two diameters. . 



are filled with darker, translucent material, 



the number of pores, and the relations of all the parts, could be very clearly 



seen. It is probable that one of these slightly larger plates with five pores, 



represents the madreporiform body, and thus faintly indicates the posterior side 



of the fossil. 



If the disc figured by Dr. Roemer is accurately represented, it would seem 

 to present a difference of even more than specific importance, from those exam- 

 ined by us, unless the difference, might be sexual. The latter suggestion, how- 

 ever, is not probable, though from all analogy we may infer that the sexes were 

 distinct in these fossils. 



From the foregoing remarks and illustrations, it will be seen that the group 

 Periscliocchinidce, which is entirely confined to the palaeozoic rocks, differs from 



* Since this was written, we have, through the politeness of Dr. Shumard of St. 

 Louis, had an opportunity to examine another fine specimen, showing the disc of this 

 fossil. In this there are four ovarian pores in three plates, and three in each of the 

 other two ; while in two of the occular pieces there is apparently a single pore near 

 one side. Hence it would appear to be the case that the individuals of this fossil pre- 

 sent unaccountable variations in the number of these pores. 



