l., 



BLASTOIDEA 



163 



Cryptohlastus meto the plates are all small and irregularly arranged. In the 

 genus Pmtremites the covering plates are modified in a singularly different 

 way : toward the centre they become increasingly elongate and spine-like, 

 suiTounding not only the central opening, but also the spiracles and anal 

 aperture, with a fringe of tapering spines, which meet over the summit in a 

 tuft-like stellate pyramid, with salient angles interradial. 



The summit in most Blastoids is surrounded by a cycle of five pairs of 

 openings ; and between the two posterior ones there is usually interposed 

 a Single additional aperture. The former were regarded by Roemer as con- 

 nected with the genital System, and were called by him " ovarian apertures " ; 

 but they are now known as the spiracles. The other opening which pierces 

 the Upper end of the posterior deltoid is the anus. 



The form and arrangement of the spiracles is extremely variable ; they 

 may be round or slit-like ; they may consist of ten separate openings, or those 

 of the same pair may be con- ^ ß 



fluent with one another. The J 



members of the posterior pair 

 may be fused with each other 

 and with the anus, in which 

 case the fifth or posterior 

 spiracle is considerably larger 

 than the others. Orbitremites, 

 Fentremites, Pentremitidea, etc. 

 (Fig. 254, A), are examples of 

 the latter case ; Orbitremites 

 having five circular orifices 

 with tube - like projections, 

 while in Pentremites and Pent- 

 remitidea the four smaller 

 spiracles are divided into two 

 compartments by the terminal 

 median ridge of the deltoids. 

 The posterior spiracle in the 

 two latter genera is divided 



A, Pentremi 



Fig. 254. 

 godoni (Defr.). Lower Carboniferous ; Alabama. 

 Upper face with ambalacral fields in various states of preservation. 

 a, Ambulacrum after the removal of lancet- and side-plates ; hydro- 

 spires exposed ; b, Lancet-plate with upper surface denuded by 

 weatliering ; c, Perfectly preserved lancet-plate bordered by side- 

 plates ; d, The same, but with transverse markiiigs of lancet-plate 

 obliterated ; e, Ambulacrum covered with pinnules (after Roemer), 

 B, Fhaenoschisma acutum (Swhy.) Lower Carboniferous ; Lancashire. 

 Upper face, enlarged ; a, Ambulacrum after removal of the lancet- 

 and side-plates ; hydrospire slits (Jiy) cutting through radials and 

 deltoids ; h, c, Ambulacra in which lancet-plates (l) only are pre- 

 served : d, e, Ambulacra intact ; lancet-plate concealed by side-plates 

 (after Etheridge and Carpenter). 



by a duplicate ridge into three 

 compartments ; of these the middle one enters the inner cavity, and the two 

 outer ones communicate with the hydrospires by means of the hydrospire 

 canal. In Troostocrinus, Schizoblastus and Cryptoblastus (Fig. 253, B) the 

 posterior spiracles are confluent with the anus, while those of the four regulär 

 sides are separated. Nudeocrhius, Mesoblastus and Acentrotremites have ten 

 separate spiracles, and a large, distinct anal aperture. The typical Codasteridae 

 (Codaster and Phaenoschisma), in which the hydrospires are exposed externally, 

 have no spiracles and no hydrospire canal. Orophocrinus (Fig. 253, B) has 

 ten elongate clefts extending along the sides of the ambulacra ; but these are 

 in reality the unclosed portions of the radial sinuses, and correspond to the 

 open hydrospire canals of Pentremites, which are apparent upon the removal of 

 the side-plates. 



The ambulacra are usually depressed below the general level of the calyx, 

 but are sometimes raised above it, or they may be placed in the same plane 

 with it. They vary in form from narrow linear to broad petaloid, and are 



