PALiEOZOIC ECHINODERMATA. 115 



narrow, of two rows of small plates, most usually of a trans- 

 versely elongate pentagonal figure, and each pierced by one 

 pair of simple pores; interambulacra wide, of five rows of 

 plates very irregular in size and shape, all the plates covered 

 with small equal granules or secondary tubercles, while the 

 row on each side adjoining the ambulacra alone bear the small, 

 smooth primary spines, one on each, the supporting tubercle 

 being small, mammillated, perforated, but not crenulated, sur- 

 rounded by a double ring and situated not in the centre, but 

 near the ambulacral edge, a little above the middle ; ovarian 

 plates pierced each with six foramina ; mouth and anus small, 

 both central. 



This genus is remarkable for the irregularity of form and size 

 of the interambulacral plates, differing in this both from Archceo- 

 cidaris and Palcechinus ; from the former it also differs in the 

 greater number of the interambulacral plates being destitute of 

 the mammillated primary tubercle, and by its small size and 

 lateral position on those plates which do bear it ; from Palcechinus 

 it differs, besides the above, in the two rows of primary tubercles 

 to each interambulacrum, &c. I at present know but one species. 



Perischodomus biserialis (M'Coy). 



Sp^ Char. Diameter (of flattened specimens) about 2| inches, 

 width of ambulacra at middle 3 lines ; width of mouth and 

 ovarian circle each about 3 lines ; granules on the five rows of 

 irregular interambulacral plates scarcely visible, the two rows 

 of mammillated and perforated primary tubercles bordering 

 the ambulacra very small ; two rows of ambulacral plates, about 

 six or seven occupying the same space as one of the interam- 

 bulacral plates of the middle of the row.. 



Some few of the ambulacral plates are wedge-shaped, pointed 

 towards the interambulacra, as in the sketch. The primary 

 spines, as far as seen, were cylindrical and smooth. 



Rare in the lower carboniferous limestone of Hook Head, Wex- 

 ford. 



(Col. University of Cambridge (anal and genital half), and 

 Dr. Griffith at Dublin (oral half).) 



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