Till' SIMM IT-PI. ATES. G9 



like those of Shumanl, were subsequently confirmed by Meek and Worthen, who 

 published some excellent figures of the summit-plates of drunntocriiuis Xvrwoodi 

 and Orophocrinus stvll if minis in the fifth volume of the ' Illinois Geological Report ' 1 ; 

 and no subsequent writer, with the exception of Hambach, has ever disputed that 

 these plates are an integral part of the organization of a Blastoid. 



In his original description of the summit-plates 2 , Shumard mentioned that, in 

 addition to those covering the central opening, there were others closing the spiracles 

 of Pentremites ; and he subsequently 3 described each of these apertures as being closed 

 " by six minute pentagonal plates, so arranged as to form a little elevation." We must 

 confess to some doubt upon the latter point, never having examined a specimen 

 which presents these characters in a sufficiently satisfactory manner ; though we 

 have certainly seen indications of what in a better state of preservation may perhaps 

 have been a group of plates closing the spiracles. White 4 , however, describes the 

 anal aperture of OropJwcrinus stcllifurmis as being completely closed by what he 

 calls "a disk of minute polygonal plates;" while Rocmer 5 and also Hall described 

 the same structure at the anal opening of Stephanocrimis, and there are indications 

 of this in one of our specimens (PI. XIX. fig. 9). 



Another form of covering to the summit of Pentremites has been mentioned by 

 Shumard 7 as occurring in P. sulcatus; but his account of it is described by 

 Hambach 8 as " insufficient and incorrect." According to Shumard a little pyramid 

 rises from the centre of the summit, with its salient angles interradial. "The base 

 of this little pyramid is joined to the superior edges of the pseudo-ambulacral fields, 

 so as to completely roof in the buccal and ovarian apertures. It consists of about 

 fifty pieces, arranged in ten series ; the first or exterior ones in each series being of a 

 triangular form, the others elongated quadrilateral. Two series of pieces stand over 

 each ovarial aperture, those of one side uniting with their fellows of the opposite side 

 at the salient angles of the pyramid." The (improved) description which is given by 

 Hambach states that this cone-shaped body " consists of little tubes running parallel 

 with each other and roofing in the summit of the calyx in a conical shape (but not 

 the central opening). They protrude through the same apertures in which the 

 hydrospires terminate ; there are about five of these tubes to each aperture, which 

 seem to correspond with the plicas of the hydrospiric sac." Here Hambach's 



1 PI. ix. figs. 2 a, 5. 



2 Journ. Acad. Nat. Sci. Philad. 1850, vol. ii. pt. 1, p. 65. 



3 Trans. St. Louis Acad. Sci. 1858, vol. i. no. 2, p. 243. 



4 Boston Journ. Nat. Hist. 1863, vol. vii. no. 4, p. 486. 



5 Archiv f. Naturgesch. 1850, Jahrg. xvi. Bd. i. pp. 369, Taf. v. fig. 3. 



6 Palaeontology of New York, vol. ii. pp. 212, 213. 



" Trans. St. Louis Acad. Sci. 1858, vol. i. no. 2, p. 244. 

 8 Ibid. 18S4, vol. iv. no. 3, p. 541. 



