146 TRANS. ST. LOUIS ACAD. SCIENCE. 



form an entirely isolated class of animals belonging neither to 

 Crinoids nor to Echinoids. 



The Pentreinites are mainly composed of two parts, i.e. the 

 calcareous shell and the softer internal organs. The former, ge- 

 nerally the best preserved part, is variable, globose, ovoid, pyri- 

 form, or clavate, composed of a number of pieces attached to a 

 little slender column by which the animal was fixed to a certain 

 place. 



The body represents a closed calcareous shell, with a few open- 

 ings on the summit, the poral openings of the ambulacral field, 

 and a small opening at the base. We observe on its external sur- 

 face five plainly marked areas, called pseudo-ambulacral fields, 

 passing from the summit, in shape of a star, more or less over the 

 sides of the body. The following table gives the number of 

 pieces characterizing this genus : 



3 Basal plates. 



5 Forked plates := radial pieces. 



5 Deltoid plates := interradial pieces. 



5 Lancet plates. 



Poral pieces, irregular in number, forming with the latter the am- 

 bulacral field, which is covered by a zigzag plicated integument 

 and pinnulas. 



Mouth central. 



Anal aperture lateral. 



Ovarian apertures 2X5 arranged around the mouth. 



The Base or Pelvis (Muenster), Plate A, Fig. i. — This is 

 composed of three pieces ; two equal in size and pentagonal, the 

 other smaller and quadrangular. Its shape varies in different 

 species ; in some it is in the form of a flat plate, in others in that 

 of a reversed cone or column. It is perforated in the centre by 

 a very small opening corresponding to that of the column. Its 

 pieces are of equal thickness, with a smooth internal surface. 

 The external surface is plain, or marked by granules, or by fine 

 stri^, with a concentrically lineated round or triangular articu- 

 lating surface for the column in the centre. This articulating 

 surface is, in many species (as sulcatus, robustiis, pyrlformis, 

 and others), surrounded by a triangular or clover-leaf shaped 

 furrow, which induced some authors* to suppose that the base 



* Lyon and Billings. 



