THE DLASTOIDEA 



Pio. XII. 



Section of ambulacrum of Orbitremites 



reached, the lancet-plates abut directly on them, without leaving any 

 pores ; the hydrospire-canc^s pass up beneath the lancet-plates, and open 

 through spiracles on either side of each A. Orbitremites, T. &. T. Austin 



(1842, generally known as Granato- 

 crinus, Hall, 1862), Carboniferous, 

 England, N. America, and (?) Queens- 

 land (Fig. XV. 3, 4). The general 

 shape and relations of the plates are 

 as in Schizoblastus and Cryptoblastus. 

 Hydrospires the same as in Crypto- 

 blastus (Fig. XII). The hydrospire- 

 canals here are surrounded by the 

 A, and pass through them to five 

 single spiracles (the posterior confluent 



Norwood}" xiQ~diaiinr"hp ,"hydrospIre-piateT with the anus) at the apices of the 



A, which sometimes project as short 



tubes. The plates roofing the mouth are minute and usually irregular. 

 Heteroblastus, Eth. & Carp. (1886), Carboniferous, England, and (?) N. 

 America, differs from Orbitremites in that the hydrospire -canals pass 

 beneath the A, and then curve outwards on either side of each A. The 

 adoral end of the A is produced upwards as a short stout process. Meso- 

 blastm, Eth. & Carp. (1886), Carboniferous, England, Belgium, (?) N. 

 America and Queensland, differs from Orbitremites in that the hydrospire- 

 canals are continued upwards over the flattened lateral portions of the 

 A, and open at the sides of the A crest. . In some species the crest is 

 almost absent, and the spiracles therefore almost single. Acentrotremites, 

 Eth. & Carp. (1883-86), Carboniferous, Somerset, is only known from 

 one specimen, but is a far more distinct genus than those just described 

 (Fig. XIII.). The general 

 form is that of Orbitremites. 

 The hydrospires do not pass 

 up into the A, but the canal 

 opens immediately on the 

 suture between A and R ; 

 there are thus ten spiracles, 

 as in Schizoblastus Sayi. The 

 anus pierces post. A, close to 

 its adoral end. The number 

 of hydrospire -folds is un- 

 known ; the inner walls form 

 a structure meeting in the 



median line, and thus like a sub-lancet ; but also passing up the sides of 

 the lancet-plate, and thus like a hydrospire-plate. The lancet-plate is 

 small and perforate ; it is covered by small side-plates and large outer 

 side-plates. 



Pentephyllum, Haughton (1859), Carboniferous, Limerick, is based 

 on a single internal cast. It is said to be unstalked, and this con- 

 dition, if obtaining, is correlated with the asymmetry shown in the 

 slight shortening of one ambulacrum and the slight curve of the 



Fio. XIII. 



Transverse section of ambulacrum of Actntrotremita 

 cllipticus. 



