378 PROCEEDINGS OF TE ACADEMY OF [1879. 



their relative position, we suppose may be equivalents of the passage 

 in the inner lancet pieces in Pentremites, designated as I in Fig. 5. 



Fig. 4. Summit of Pentremites, showing the deltoid pieces and their appen- 

 dages hidden from view by the test in perfect specimens ; a = anal 

 opening ; h = hydrospires ; o = inner Avail or floor of the passage (so- 

 called ovarian openings) leading to the hydrospires ; the outer wall, 

 which is a part of the pseudambulacrum, having been removed from the 

 specimen ; d = the outer or visible portion of the deltoid pieces equiva- 

 lent to d in Fig. 2 ; I = plates lying directly below the lancet piece, 

 with tubular passage running lengthwise through them (see I in Fig. 

 5) ; b = passage formed by the edges of two deltoid pieces and the in- 

 ner lancet piece, externally covered by the pseudambulacrum equiva- 

 lent to the opening at the base of the arms in Paleocrinoids. 



This figure is a representative of the structure of this portion of the 

 Blastoid, as ascertained by examination of a number of specimens, 

 some in which parts of the test have been removed, others in form of 

 polished sections. It is based upon observation, not imagination, and 

 this figure, in connection with No. 5, will, we hope, enable the reader 

 to understand a construction alwa} 7 s difficult to explain, and which is 

 now, for the first time, correctly figured, as we believe. We have not 

 attempted to illustrate the details upon which our figure is based, be- 

 cause our object at present is only to point out certain affinities between 

 some forms of Palseocrinoidea and the Blastoidea. 



Fig. 5. Cross section of Pentremites pyriformis Say (at one-half the height 

 of the ambulacrum, one ray of the latter being removed). 



f= the halves of the forked plate ; L = lancet pieces ; 1= inner lan- 

 cet pieces; p = pore pieces ; m = ambulacral groove and food passage 

 covered in perfect specimens ; e= pores on the ambulacral field. 



Figs. 6, 7, 8. Hydrospires of Caryocrinus ornatus. 6. Surface view, show- 

 ing the openings through the test. 7. Representing the course of the 

 flat internal tubes. 8. Transverse section. 



(Note. The figures on this plate are more or less enlarged.) 



