CARBONIFEROUS FOSSILS OF IRELAND. 173 



licxagonal plates ; one of these rows always becomes obsolete before reaching the anus, thus producing an irre- 

 gularity in the shape of the adjoining plates; surface of the plates irregularly studded with minute tubercles. 



This fine species is distinguished by its spherical form, and the number and irregularity of the interambu- 

 lacral plates. I am not acquainted with any other Echinite in which the interambulacra are dissimilar ; in the 

 present species, however, some of the interambulacra have six rows of plates, while others have seven. 



ECHINOCKINUS. AgasS. 



Gen. Ch. — Interambulacra composed oC thrfc or more rows of plates, those on each side, next the ambu- 

 lacra, pentagonal, those of the intermediate rows hexagonal, as in Paleec/iinus ; each plate having in the centre 

 one large, perforated tubercle, surrounded by an elevated ring, as in Cidaris, each of which tube-rcles bears a 

 large, mobile, generally muricated spine. 



It is a singular circumstance that, except Professor Agassiz, every author who has hitherto treated of the 

 Echinodermata of the mountain limestone, should have referred the hexagonal plates with the above characters 

 to the genus Cidaris, when a glance at the recent or Oolitic Cidarites, would be sufficient to shew that in their 

 entire framework there is not one hexagonal plate, both the ambulacra and interambulacra being composed 

 each of two rows of pentagonal plates only, while in the present genus, as in Palcechinus, their interambulacra 

 must have been composed of more than two rows, as is obvious from their hexagonal form : the large, perforated 

 tubercle, however, is precisely in accordance with that of the true Cidaris, as is also the mode of attachment 

 and general character of the large spines with which both genera are armed. Thus, as we have seen, the Echini 

 of the mountain limestone to differ remarkably as a group from the Cainozoic and Mesozoic genera, in the com- 

 plexity of their interambulacra; so do we find the Cidarites of the Palasozoic rocks to differ in the same cha- 

 racter, both from the recent and newer fossil forms : while the relation which the recent Cidarites bear to the 

 recent Echini is precisely equivalent to that which Echinocriniis holds to Palcechinus. I had long ago distin- 

 guished this genus in my MSS. under the name of Arc/ircocidaris, subsequently Professor Agassiz announced 

 his intention of forming the genus Echinocrinus for the Cidaris Nerii, &c. 



EcHiNocRiNus MuNSTEEiANUs ? Koninck. SP. (PI. XXVII. fig. 2). 



Sp. Ch. — Spines cylindrical, with about twenty-three longitudinal, oppositely muricated ribs. 



This beautiful little spine is at once distinguishable from any other living or fossil species with which I am 

 acquainted, by the great number of its minutely denticulated ribs ; it is very rare. The fragment figured mea- 

 sures half an inch in length; breadth two lines. The figure I have given was lithographed nearly three years 

 ago, previous to its description by M. de Koninck. 



Echinocrinus glabrispina. Pldl. sp. 



Cidaris glabrispina. Phil. Geol. York. 



I have seen two or three smooth spines o? Echinocrinus about one inch in length, and tapering gradually 

 to a point, which, I suppose, are referrible to this species. 



Echinocrinus triserialis. M'Coy. (PI. XXVI. fig. 1). 



Sp. Ch. — Large mobile spines, triangular; each of the angular ridges armed with a row of strong, tooth- 

 like spines; intervening spaces smooth. 



The spines of this species difier from all I am acquainted with, in having but three longditudinal rows of 

 spines, or denticulations, the other species of the limestone having four or five rows ; the portions of the sp)ine 

 not muricated are smooth in this, but strongly striated longitudinally in the other species ; it also differs in its 

 triangular form, the sides being nearly flat and almost equal. Length of specimen figured, imperfect at each 

 end, one inch eight lines, diameter one and a half lines. 



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