I11K EYDB0SPIBE8 AM) 8PIBACLES. 109 



he an hydrospire-plate, between which and the radials there are numerous pores, as in 

 the closely allied genus Cryptoblastua, and also in Mesoblastus and Granatocrinut 

 (PL IV. fig. 4 ; PI. VI. fig. 10; PI. VII. fig. 15 ; PI. X. figs. 11, 12). 



The anal opening of Acentrotrernites is altogether distinct from the spiracles i f its 

 interradius, as it is much nearer the mouth, piercing the proximal end of a deltoid 

 (PI. XIII. fig. 19). In Cryptoblastus, however, the anus is confluent with the 

 spiracles at its sides (PI. VII. fig. 14). These are not constructed in quite the same 

 way as those of Acentrotrernites, as the pointed proximal end of the lancet-plate takes 

 part in their formation (PI. VII. figs. 14, 15). A wide median ridge rising from the 

 crenulated apex of each deltoid separates two spiracular openings which are appa- 

 rently constructed upon the same type as those of some Pentremites, more especially 

 /'. Burlingtonensis, viz., leading into an hydrospire-canal which is roofed in by side 

 plates (PI. VII. fig. 14) ; but the removal of the side plates does not expose the 

 concealed parts of the deltoids which form the floor of the hydrospire-canal, as in 

 Pentremites (PI. I. figs. 5-7 ; PI. XII. fig. 13), for at the central end of the ambu- 

 lacrum the lancet-plate comes into direct and continuous contact with the deltoid 

 without leaving any intervening pores, although the side plates rest upon it as usual 

 (PI. VII. fig. 15). The same is the case in Acentrotrernites and in Orojjhocri/nis 

 stelliformis (PI. XL fig. 9; PI. XIII. fig. 19; PI. XV. fig. 11); but it is the 

 natural result of the position of the spiracles at or near the distal ends of the 

 deltoids, so that the hydrospire-canals do not extend beyond them towards the 

 mouth, and no marginal pores are wanted at the proximal ends of the ambulacra. 



In Cryptoblastus, however, the pointed proximal end of the lancet-plate is slightly 

 notched, and there are corresponding notches in the deltoid plates at its sides, so as 

 to form the spiracles which lead downwards beneath the lancet-plate into the 

 hydrospire-canal. But this is altogether covered up by the lancet-plate along the 

 whole length of the deltoids with which this plate is in close contact. Beyond the 

 radio-deltoid suture, however, it comes away from the side of the sinus, and so makes 

 room for the hydrospire-plate which forms the inner boundary of the hydrospire- 

 canal ; and the canal thus opens to the exterior through the series of pores which 

 are formed between the hydrospire-plate and the radials, just as in Mesoblastus and 

 Granatocrinus (PI. IV. fig. 4; PI. VI. fig. 10 ; PI. VII. fig. 15 ; PI. X. figs. U, 12). 



Cryptoblastus melo is taken by Hambach ' as the type of his second division of the 

 genus Pentremites, in which he also includes Schizoblastus Sayi, Pentremites Burling- 

 tonensis, and Mesoblastus crenulatus. He says that it " would comprise all those 

 species in which the deltoid pieces are very broad, the lancet-pieces very narrow, and 

 the sinus for the formation of the spiracle-openings in both deltoid and lancet-pieces 

 very little ; the zigzag plicated integument corresponding to the narrow ambulacral 

 field is not wide enough, so as to surround these openings fully, hence they have to 

 1 Tran3. St. Louis Acad. Sci. 1884, vol. iv. no. 3, p. 544. 



