Summit Structure of Pentremites. 487 



ined is a little below the plane of the general surface. 

 These plates are so small that the number covering an 

 aperture of half a line in diameter is eighteen or twenty. 

 Their size and arrangement are somewhat irregular. The 

 lancet pieces are apparently longitudinally perforate, and 

 rest in broad grooves in the radial plates, at their outer 

 ends. A narrow groove runs along the centre of the upper 

 side of each. These grooves meet together at the sum- 

 mit aperture, and are bordered on each side by the poral 

 pieces. They are neatly filled by a compound series of 

 minute plates, which closely connect at the summit with 

 five small plates, arranged like a five-pointed star, with 

 the points touching each of the upper ends of the inter- 

 radial plates, thus completely covering the summit aper- 

 ture which weathered specimens show to exist beneath, 

 formed by the slight truncation of the interradial plates. 

 The stellate series of five plates at the centre being an 

 integral part of the integument which fills the grooves, 

 with their position, forbids the supposition that they were 

 a movable covering for the mouth, which evidently was 

 not situated here, but in the same aperture with the anus. 

 The filling of the central grooves of the pseudambulacral 

 fields, and the covering of the central summit aperture, 

 are essentially the same in this species as in P. Nor- 

 woodii, and doubtless served the same purposes, as pre- 

 viously suggested. 



This species, as before remarked, is not properly refer- 

 able to any of the groups established by Dr. Rcemer, but 

 in some respects approaches Cadaster ; more, however, 

 in its summit apertures than its resemblance to the Car- 

 boniferous forms of that genus. 



If the suggestion that the thin slits near the summit, on 

 each side of the pseudambulacral/fields, are equivalent to 

 the ovarian or siphonal apertures of other Pentremites is 

 correct, it may not prove fruitless to look for similar ones 



