90 PROCEEDINGS OF THE ACADEMY OF [1887. 



ering tlie side grooves, which are arranged as regularly as those of 

 the main grooves, have been traced by us as far as the fifth side 

 pieces, but may have extended farther down. In spite of their 

 small size, the plates are very distinct in our specimens, those of the 

 upper row resting closely against the spines. 



Messrs. Etheridge and Carpenter have given two figures showing 

 the summit of Pentremites in two different conditions of preservation, 

 both of them from specimens belonging to us. The first figure of P. 

 sidcatus, (PI. I, fig. 8.), gives a somewhat incorrect impression of the 

 external appearance of the pyramid, owing to the fact that the 

 spines composing it have been irregularly broken ofi* a little way 

 above their bases, so that what there appear as plates are really the 

 cross sections of the spines. The fracture, however, is not regular, 

 nor at right angles to the long dimension of the spines, so that the 

 figure does not correctly represent either the real form of the cross 

 sections, or their relative positions. We may observe also that the 

 figure does not give the central portions. By applying a little ani- 

 line coloring matter we have been able to see the sutures indicating 

 the broken ends of spines, but the fracture is so irregular that the 

 arrangement cannot be distinguished. In their other figure on PI, 

 V, fig, 28., which gives a good idea of the form and character of 

 the spines composing the pyramid, some of the pinnules are pre- 

 served overlapping the spines and resting upon them, showing in 

 marked contrast the diflTerence between the two structures. 



After quoting and commenting upon Shumard's and Hambach's 

 'descrijitions of the pyramid in P. sulcatus, and having before them 

 the original sj)ecimens represented in the above mentioned figures, 

 Etheridge and Carpenter ^ give their intrepretation of the facts as 

 follows : 



" Mr. Wachsmuth has sent us a fine specimen, which may per- 

 haps throw some light on this difficult question, (PI, I, fig. 8.) The 

 peristome and spiracles are almost completely covered by what seems 

 to be the base of the little pyramid described by Shumard." The 

 upper part of the pyramid described by Shumard and Hambach 

 "seems to us to be constructed by the proximal pinnules, as in the 

 specimen represented on PI. V, fig. 28. In Mr. Wachsmuth's ex- 

 ample of P. sulcatus, however, these proximal pinnules are not pre- 

 served, and the angles of the pyramid extend outwards towards the 

 pointed ends of the visible parts of the deltoids. At two of these 

 angles there seem to be indications of a double series of plates above 



1 Catalogue of the Blastoidea, p, 70. 



