168 CATALOGUE OF THE BLASTOIDEA. 



limbs nearly of equal length with, or longer than, the bodies, flat sided ; sinuses 

 wide, with sharp margins; lips simple, not projecting; interradial sutures on the 

 flat sides, or sometimes in very slight concavities. Deltoid plates small, sharply 

 lanceolate, externally not reaching quite to the summit, surface slightly concave. 

 Ambulacra subpetaloid, flat, depressed below the edges of the sinuses ; lancet-plate 

 usually equal in width to the two series of side and outer side plates combined ; side 

 plates twenty to forty in number ; inner line of sockets excavated out of both lancet- 

 plate and side plates ; pores large and triangular. Five to seven hydrospire-folds 

 on each side. Spiracles closely surrounding the mouth, rhombic oval. Mouth 

 moderately large. Ornament of fine lines as usual. 



Remarks. In describing P. piriformis, Say remarked — " Found in plenty in 

 Kentucky, in the same localities, and intimately intermixed with the succeeding 

 species (i. e. P. Godoni) ; it may be readily distinguished by the gradual attenuation 

 of Ihe pelvis and contiguous parts, from the tips of the emargination of the scapulas, 

 to the origin of the column 1 ." We are unable to describe the difference between 

 this species and P. Godoni in more expressive language. 



Messrs. Dujardin and Hupe, following Roemer, were inclined to regard this 

 species as a variety of P. Godoni, or at any rate as very closely related to it. We 

 have investigated this question by the aid of a large amount of material, and do not 

 find such intermediate gradations as would warrant a union of the two species. 

 The minute structure of the ambulacra of P. pyriformis was carefully studied by 

 the late Mr. Billings 2 , whose description of it has been quoted already 3 , and is well 

 illustrated by figs. 1-3 on PI. I. 



P. piriformis varies almost as much in the outline of the calyx as does P. Godoni. 

 The largest condition appears to be the form which Hall has designated as P.symme- 

 tricus (PI. II. fig. 24). In this variety that portion of the calyx lying between the 

 summit and the apices of the ambulacra is twice the height of that between the latter 

 and the stem facet. A similar disproportion exists in our figs 25, 26 of the same 

 Plate, but on glancing at figs. 27-30 it will be observed that the basal portion of the 

 calyx gradually becomes elongated, until in the form represented by fig. 30 the 

 proportions are as nearly as possible equal. This point is also well seen in Eoemcr's 

 figure of P. piriformis 4 . From Say's expression 5 " pelvis gradually attenuated," we 

 believe this to be the typical form of the species. The peripheral diameter of the 

 calyx also appears to vary to some extent. It is greater in proportion to the height 

 in the original of fig. 27 than in any of our other specimens. 



Localities and Horizon. Chester, and Prairie du Long, Illinois ; Cumberland 



1 Journ. Acad. Nat. Soi. Thilad. 1825, vol. iv. pt. 2, p. 294. 



2 American Journ. Pci. 1870, vol. 1. p. 228. s Antea, p. 60. 

 * Arehiv 1'. Nnturgc-ich. 1 S ."H, Jahrjr. x\ii. lid. i. t. f>. f. 9rt. 



5 Journ. Acad. Nat. Sci. Philad. 1825, vol. iv. pt. 2, p. 294. 



