194 



ECHINODERMATA PELMATOZOA 



SUB-KINGDOM III 



the climax of Blastoid development takes place in the Sub-Carboniferous of 

 North America. Some of the beds of the Kaskaskia Group are fairly charged 

 with their remains, which as a rule are excellently preserved ; but above this 

 horizon no traces of them have as yet been discovered. Nineteen genera, 

 comprising upwards of 120 species, are recognised by Etheridge and Carpenter 

 in their admirable monograph. 



Order 1. REGULARES. Etheridge and Carpenter. 



Pedunculate Blastoids with a symmetrical base, in which the radials and ambulacra 

 are all equal and similar. 



Family 1. Pentremitidae. d'Orbigny. 



usually convex, and often much elongated. Spiracles five, but sometimes more 

 or less completely divided by a median septum, and bounded proximally by the upper- 

 most side-plates. Lancet-plate either entirely visible or partially covered by side-plates 

 which extend to the margins of the ambulacra. Hydrospires concentrated at the lowest 

 part of the radial sinus. Devonian and Sub-Carboniferous. 



Pentremites, Say. Calyx usually ovate or pyriform, with elongate, sub- 

 truncate base. Ambulacra broad, sub-petaloid. Lancet-plate wholly exposed, 



and resting below on an under lancet-plate. 

 Side-plates and outer side-plates' numerous, 

 the former abutting against the edges of the 

 lancet-plates. Hydrospires three to nine ; 

 spiracles single, or occasionally double ; the 

 two of the posterior side confluent with the 

 anus, and forming with it a single large 

 orifice. Summit covered by numerous spines, 

 placed closely against one another so as to 

 form a pyramid, which completely covers 

 the summit and the greater portion of the 

 spiracles. Excessively abundant in the Sub- 

 Carboniferous of North America (St. Louis 

 and Kaskaskia Groups), but not identified 

 in Europe. P. Godoni, Defrance, and P. pyri- 

 formis, Say, are the most familiar species. 



Pentremitidea, d'Orb. Calyx clavate- 

 pyramidal, with elongate, usually conical 

 base, and truncate or convex upper face. 

 Ambulacra narrow, short; lancet-plate more 

 or less completely concealed by side-plates. 

 Deltoids very small, generally confined to the upper face of the calyx, and 

 seldom visible in a side-view. Spiracles and hydrospires as in the preceding. 

 Lower and Middle Devonian; Eifel, Ardennes, Spain, Great Britain, and 

 North America. P. Pailletti, de Vern. ; P. Eifelianus, Eoemer ; P. clavatus, 

 Schultze. 



Mesoblastus, E. and C. Calyx oval to globose, with concave to protuberant 

 base. Radials long, deltoids small, short, unequally rhombic. Ambulacra 



FIG. 319. 



Pentremites 

 Co'/niii, Defr. 

 Sub-Carbon- 

 iferous ; 111. 

 (Nat. size.) 



FIG. 320. 



Pentremites sulcatus, 

 Roein. Sub--Garbonifer- 

 ous ; 111. A, Summit as- 

 pect. B, Base. 





