Summit Structure of Pentremites. 483 



PKNTREMITKS XOKWOOIMI Owen and Shumur.l. 



The interradial pieces of this species are small, and so 

 arranged that tiiey join each other at the sides of their 

 upper ends, being separated from each other through the 

 greater part of their length by the interposition of the 

 pointed ends of the lancet pieces, and slightly truncated 

 at the upper ends, leaving a subpentagonal aperture which 

 is probably reduced in size by the insertion of other small 

 plates, and is also covered by an integument of small 

 plates which will be presently noticed. 



Within the body, along the sides of each of the lancet 

 pieces, at the termination of the pores, run shallow grooves, 

 which terminate in the so-called ovarial apertures. These 

 apertures are five in number, including the anal aperture, 

 and are pierced through the solid substance of the inter- 

 radial plates, the perforations dividing within the sub- 

 stance of the plates ; each communicating with two of the 

 forementioned grooves, one from each of the adjacent 

 rays; and are, except the anal aperture, prolonged above 

 into small, rather short, tapering, erect or converging tubes. 

 These tubes are not composed of small plates, but are in- 

 tegral parts of the interradial plates, and being very thin at 

 their upper ends, their condition seems to render it doubt- 

 ful if their apertures ever possessed a plated covering. 



The anal aperture is larger than the others, into which 

 one of the so-called ovarian apertures from each of the 

 adjacent rays opens, and is bordered on the outer side by a 

 solid projection of a part of the interradial plate, of about 

 the same size and height as the forementioned tubes. 



The pseudambulacral fields are composed of the ana- 

 logues of the different elements of the corresponding parts 

 in the typical group of Pentremites, but in common with 

 all the elliptici, they present some important modifications. 

 The poral pieces, and supplementary poral pieces are small 



