226 CATALOGUE OF THE BLASTOIDEA. 



denies the existence of these summit-plates, and has given various explanations of 

 them, which we have already discussed on pp. 66-69. 



He has further described, under the name of Pentremites Potteri \ a fossil which 

 appears to us to be nothing but a small variety of Schizoblastus Sayi. Almost every 

 character to which an intelligible reference is made in the specific diagnosis of 

 P. Potteri is mentioned in that of S. Sayi previously given by Shumard, and is to be 

 found in one or other of the different varieties of this species. The only difference 

 between the two types, according to Hambach, is that P. Potteri " is generally 

 smaller, .... the pelvis is not concave, the fork-pieces not arched at the junction 

 of the deltoid piece, and this latter is not elevated and ornamented with transverse 

 striae, but depressed and granulated." Now, in the first place, we have before us 

 specimens of S. Sayi in which the base is respectively a little concave, almost flat, or 

 slightly protuberant. The radio-deltoid suture is more or less distinctly arched in 

 all of them, and it is represented as arched in Hambach's own figure of P. Potteri, 

 although he makes a positive statement to the contrary. His description of the del- 

 toids of P. Potteri corresponds exactly to the condition of these plates in S. Sayi ; 

 and neither Shumard nor any later writer has described the deltoids of this type as 

 being "elevated and ornamented by transverse striae." 



Hambach's description of the spiracles of P. Potteri appears to us to be either 

 unintelligible or incorrect. It runs as follows : — " Ovo-spiracle apertures and anus 

 very small, and kept separate by the broad deltoid pieces ; they are surrounded by 

 the zigzag plicated integument in this as well as in other species." But if the two 

 slit-like spiracles of each interradius are kept separate by the broad deltoid pieces, 

 how can they be " surrounded by the zigzag plicated integument " 1 This struc- 

 ture is elsewhere described by Hambach as covering the whole ambulacral field ; 

 but unless it also extends on to the "broad deltoid pieces, how can it "surround" 

 the slit-like spiracles at their sides? As a matter of fact, the appearance described 

 by Hambach under this name is merely the crenulation of the ambulacral groove 

 and of its lateral branches, as we have already explained on pp. 58, 59 ; and as it 

 does not occur at all on the deltoid side of the slit-like spiracles, it can hardly 

 be said to "surround" them, either in Schizoblastus Sayi (PL 111. figs. 1-3) or in 

 other species. 



Localities and Horizon. Burlington, Iowa; Missouri : Upper Burlington Limestone, 

 Subcarboniferous. 



Schizoblastus mklonoides, Meek & Worthen, sp. 



(PI. VI. figs. 15, 16.) 



Granatocrima melonoidet, Meek & Worthen, Proc. Acad. Nat. Sci. Philad. 18(51), p. HH. 



1 Trans. St. Louis Acad. Sci. 1880, vol. iv. no. 1, p. 150, pi. B. fig. 4. 



