370 PALAEONTOLOGY OF ILLINOIS. 



conical above ; under surface flat, or slightly concave, rough ; 

 upper surface smooth, at summit somewhat ridged with short 

 rows of enamelled tubercles ; microscopic structure showing a 

 congeries of irregular, vertical, prismatic columns. 



Of these specimens the larger one, represented by fig. 5, 5a, plate ii, is evi- 

 dently much worn on the upper surface, and is so massive and strong that it 

 seems something of a stretch of the imagination to consider it a dermal tuber- 

 cle. It has, however, a strong generic resemblance to the specimens of Petro- 

 dus occid entalis, figured in vol. 2, pi. iv, especially to that represented in fig. 

 16, and is also so closely allied to the smaller specimen now figured (pi. iii, fig. 

 6), that there seem to be no good grounds for separating them. 



As has been stated in our description of P. occidentalis (1. c.), there are 

 apparently good reasons for considering that to be the dermal tubercle of some 

 Plagistomous fish, and the reasons given for that conclusion are to a certain 

 degree applicable to the specimen before us. The microscopic structure is 

 certainly different from that of any of the teeth which have come under our 

 notice. The mass of this fossil is composed of contiguous, prismatic columns, 

 which run through from the upper to the lower surface; in the larger speci- 

 mens apparently solid and homogeneous in structure, while in the smaller they 

 form polygonal tubes which give a reticulated marking to the tipper surface. 

 These prisms may be considered to be the Lomologues of the caleigerous tubes 

 which by their ends mark the enamelled surface of most of the placoid teeth 

 that have been described, but in all the species of Psammodus, Deltodus, etc^ 

 these tubes are distinctly separated at their superior extremities, while they 

 inosculate below. The difference which the two forms of structure exhibit will 

 be best understood if we compare the calcicerous tubes of JPsammodus, Cocli- 

 liodus, etc., with Syringopora, while the tubes or prisms of the specimens be. 

 fore us may be compared to Favosites. 



Formation and locality : Burlington limestone; Burlington, Iowa. 



GENUS ASTEROPTYCHIUS, Ag. 



ASTEROPTYCHIUS TRIANGULARIS, N. and W. 

 PI. ii, fig. 4. 



SPINE short, robust, with a nearly equal-sided, triangular 

 section, the sides concave ; anterior keel strong, sharp, and 

 smooth ; lateral surfaces marked with 5-6 nearly equal, smooth, 



