w. keeping on the paleozoic echini. 39 



4. Arch^ocidakis. 

 Archjsocidaris Ueii (Fleming). PI. III. figs. 14-18. 



A specimen found by Professor Hughes at Hafod-y-calch, Corwen, 

 possessing sixteen large plates but slightly displaced from their 

 natural position, indicates that this Urchin possessed as many as five 

 ranges of interambulacral plates * in each area. These plates are 

 very thin, and imbricate from the central ranges outwards (fig. 17) as 

 in Perischodomus. The dorsal and ventral edges were more firmly 

 united by a sort of hinge (fig. 16), the edge of one fitting into a groove 

 in the other, as described by Mr. J. Young f , from which description 

 it also follows that the ambulacral plates were overlapped by the 

 contiguous interambulacrals. 



Part of an ambulacral range is preserved on one side, showing 

 the plates to be very irregular and perforated by two large pores. 

 A very good idea of the aspect of these plates may be got by ima- 

 gining a couple of corallites cut from the Dudley chain coral, 

 Halysites catenulatus. The only interambulacral plate which is 

 contiguous to this series of plates is not pentagonal, but hepta- 

 gonal, with rounded angles (fig. 15); this was explained by the dis- 

 covery of a broadly wedge-shaped scale-like ambulacral plate (fig. 18), 

 the head of the wedge being produced and rounded to fit into the 

 space made by cutting ofT the angles of the interambulacral plates 

 as above described. Such an arrangement would help to keep the 

 plates in position during flexures of the test. 



Associated with the plates and large thorny spines of this species 

 the small secondary spines may nearly always be found. These re- 

 semble those of Perischodomus in appearance ; they are tubular and 

 coarsely ribbed, almost as coarsely as the larger spines, but possess 

 no thorns upon them. 



Opinions have differed widely as to the value of the group 

 Perischoechinida in classification. By Prof. McCoy it was placed as 

 a distinct order of the Echinodermata, while Pictet includes its mem- 

 bers in the family Cidaridae. But though no structure is known in 

 them which is not represented in the Echinoidea, they are well 

 distinguished from all the rest of the order by the greater number 

 of ranges of interambulacral plates, and, so far as is yet known, also 

 by the fact that when the plates of the test overlapped, this over- 

 lapping was in the opposite direction to that of the flexible Echini 

 of the more modern group, and, again, by the greater number of 

 pores in the apical series of plates, a character indicating other 

 physiological variations £. They are equally well marked off in time — 

 all the Perischoechinidae having disappeared at the end of the Palaeozoic 

 period, before any of the modern types of Echini had appeared. 



Prof. Wyville Thomson's remark in connexion with the Echino- 



* A. Wortheni (Hall, Geol. Iowa, vol. ii. p. 700) has four rows of plates in 

 each interambulacral area. 



t Geol. Mag. vol. x. p. 302. 



{ Double genital pores occur in the genus Cidaris (Agassiz, Kev. Echin. 

 p. G47). 



