234 CATALOGUE OF THE BLASTOIDEA. 



only eight on the summit of P. Burlingtonensis, as the posterior pair are confluent 

 with the anus, so that there are not ten, but only nine, " distinctly visible openings " 

 besides the mouth. We doubt whether Hambach has ever seen P. crenulatus, and 

 suspect that his knowledge of its summit is confined to Roemer's figure l . But even 

 this does not show ten spiracles. There is a large undivided anal spiracle, and four 

 others which are partially divided, as in our PL VI. fig. 8 ; but we think that even 

 Hambach would hesitate before he spoke of " ten distinctly visible openings " in the 

 specimen represented on PI. IV. fig. 1. 



On the other hand, there are really nine openings in his type species Pentremites 

 melo (PI. VII. fig. 14), viz. four pairs of ordinary spiracles and one anal spiracle. 

 Put the diagram which Hambach gives of the summit of this species shows ten 

 openings besides the anus, whereas there are really only eight. He has inserted a 

 pair of spiracles in the posterior interradius which have no existence in fact (PI. VII. 

 fig. 14), but are necessary to his system of classification. The five species which he 

 names as belonging to his second division of the genus Pentremites represent, in our 

 view, four different generic types — viz. Cryptohlastus, Scltizoblastus, Mesoblastus, and 

 Pentremites, and we believe that those palaeontologists who have read the preceding 

 chapters will not think our system of classification so "impracticable" as it is said 

 to be by Hambach. 



Localities and Horizon. Burlington, Iowa; Hannibal and St. Louis Co., Missouri; 

 near Monmouth, Illinois: Lower Burlington Limestone, Subcarboniferous. 



Genus ACENTROTREMITES, E. & C, 1883. 



Mitra, Cumberland (pars), Reliquiae Conservatce, 18.20, p. 31. 

 Acentrotremites, E. & C, Ann. & Mag. Nat. Hist. 1883, vol. xi. p. 232. 



Gen. Char. Calyx elliptical, with a broad pentagonal base largely formed by the 

 radials, which also take up three fourths of its height. Deltoids unequally rhombic, 

 each notched by two spiracles at the ends of the radio-deltoid suture. Anal opening 

 situated close to the peristome in the posterior deltoid. 



Remarks. We established this genus in 1883 for a very singular form which, in 

 some respects, is intermediate between Cryptohlastus and Schizoblastus. It resembles 

 the type of the latter genus (S. Sayi) in the presence of distinct spiracles in the 

 posterior interradius. But the anal opening, instead of being between the spiracles 

 as in that species, or farther from the peristome as in S. Missouriensis, is situated at 

 the central end of its deltoid quite close to the peristome (PI. XIII. fig. 1'.)), while 

 the ten spiracles notch the outer ends of the deltoids where the radio-deltoid sutures 

 meet the ambulacra. There are, therefore, no hydrospire-pores along the ambulacra! 

 edges of the deltoids. This is also the case in Cryptohlastus (PI. VII. fig. 15), but it 



1 Archiv f. Naturgesch. 1851, Jahrg. xvii. Bd. i. Taf. iv. f. 15 c. 



