482 Observations on the 



the retreating angles correspond to the centres of the 

 pseudambulacral fields. The base of this little pyramid is 

 joined to the superior edges of the pseudambulacral fields, 

 so as to completely roof in the buccan and ovarian aper- 

 tures. 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 ovarian aperture, those of 

 one side uniting with their fellows of the opposite side at 

 the salient angles of the pyramid." 



Dr. Shumard further expresses the opinion that all the 

 summit openings of Pentremites were closed, and in the 

 second Missouri Report again remarks, — "I have but 

 little hesitation in advancing the opinion that all the 

 Pentremites included at present in the group eUiptici of 

 Roemer will be found like Eleacrinus deficient in a cen- 

 tral summit opening." 



The difference in the character of these summit cover- 

 ings which Dr. Shumard has shown to exist in the same 

 group {Jloreales) is remarkable, but may not have been 

 inconsistent with a similarity of structure of other parts. 

 The following observations also show much diversity in 

 the structure of the superior parts of different species of 

 the group e/liptici, yet all seem to difler materially from 

 those of the Jloreales. 



Besides the numerous examples which my own cabinet 

 affords, I have been favored with access to the extensive 

 collections of Rev. W. H. Barris, Mr. Charles Wachsmuth, 

 B. J. Hall, Esq., and Dr. Otto Thieme, of Burlington, Iowa, 

 which enables me to add to the foregoing interesting ob- 

 servations of Dr. Shumard the results of my own upon 

 the summit structure of the following well-known species 

 ' of Pentremites, as well as further observations on the 

 structure of certain parts of Crinoids. 



