DESCRIPTIONS OF THE SPECIES. 1G3 



are of about equal length; sinuses moderately wide, with bead-like margins; lips 

 simple, and projecting but little; interradial sutures in concavities. Deltoid plates 

 acutely and unequally rhombic, with concave surfaces, the hinder margins shorter 



than those nearer the summit, but in tin- young form the plates are simply lanceolate. 

 Ambulacra convex, subpotaloid ; lancet-plates of less width than the combined side 

 plates, which number about forty, but in the young state are generally from ten to 

 twenty. Four hydrospire-folds on each side Spiracles triangular-ovate, closely 

 placed round the mouth, which is small. Ornament of fine close stria" arranged as 

 usual, hut the stria? on the hands along the edges of the sinuses are stronger than 

 the others. 



Remarks. P. conoideus taken alone as originally described by Prof. J. Hall would not 

 readily be mistaken for any other species. We follow Prof. R. P. Whitfield, however, 

 in considering P. conoideus and P. Koninckanm as respectively the older and younger 

 conditions of one and the same species 1 . He says, " The specimens described under 

 this name (i.e. Koninckana -)are probably the young of P. conoideus, Hall. All the 

 variations between them are in the direct course of development by additional growth. 

 The short base, almost flat in the older specimens, is one of the most prominent 

 characteristics of P. conoideus, but among a large number of small specimens of the 

 species from the different localities none are found with flat bases, hut as they increase 

 in size this feature becomes more and more apparent." 



We have had the advantage of studying a very large series of both conditions of 

 P. conoidt us, and we altogether concur in the above remarks. In support of the 

 union of Hall's two species we give a series of outline drawings of the younger 

 stages of his P. Koninckana (PI. II. figs. 19-23), leading by gradations to the conoidal 

 outline of P. conoideus (figs. 16-18). As the shell grew the base shortened, and the 

 ambulacra increased in length, whilst at the same time the summit became more 

 acute. 



As regards the adult stage, we again quote from Prof. "Whitfield- — "This species 

 differs from any of the many described from the Carboniferous rocks of America in 

 the conoidal form of the adult specimens, though young shells are common which 

 present nearly the form of P. pyriformis, Say, and others that closely resemble 

 P. elegans and P. symmctricus, Lyon" 3 . 



Dr. Shumard described a somewhat different arrangement of the apical covering- 

 plates in this species from that usually met with in the Blastoidea. Instead of the 

 complete dome, composed of small but irregular polygonal plates, which has been 

 usually noticed by authors, Shumard described each aperture at the summit as 

 closed by six pentagonal plates, viz. a central large one and five smaller ones sur- 

 i Bull. American Mus. Nat. Hist. 1882, vol. i. no. ::, p. 44. 



2 We do not quite understand why Hall used a feminine termination for this specific name. 



3 Lot. cit. p. 45. 



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