PALAEOZOIC ECHINODERMATA. 113 



opposite ends of the ventral disc — all agree in having their case 

 made up of twenty vertical rows of plates, ten ambulacral and ten 

 interambulacral. This is not only the most persistent character 

 of the entire group, but the number becomes of extreme interest 

 when, with Agassiz and Valentin, we view the globose test of the 

 sea-urchins as a mere modification of the same parts which we find 

 in a 5 -rayed starfish, — an ideal division of the mesial suture con- 

 necting the two rows of plates in each interambulacrum of the 

 former, giving at once the ambulacra, lateral ossicles, and other 

 characters of the latter. The Echinites of the palaeozoic rocks 

 however are constructed on an entirely diiferent plan, having three 

 or more rows of interambulacral plates, instead of two as in those 

 of the newer rocks and existing seas; as therefore those sea-urchins 

 differ from all of the order Echinida in the great number of rows 

 of plates in the test, usually having an odd number of rows in the 

 interambulacra, and the consequent impossibility of theoretically 

 dividing them at the sutures into five equal parts, I would pro- 

 pose to form a peculiar order for their reception under the above 

 title, indicating the complexity of their structure. I first drew 

 attention to the structural peculiarities of those fossils in 1844 

 in my ^ Synopsis of the Garb. Limest. Fossils of Ireland ^ (p. 171 

 to 174), where 1 gave the generic characters of the genus Palce- 

 chinus (proposed in manuscript by my friend Dr. Scouler), and 

 described and figured several species having from three to five 

 rows of plates in the interambulacra. In the same work I stated 

 that the plates of the so-called Cidarites of the carboniferous 

 period being hexagonal was a proof that they too must have had, 

 like the Palachini, more than two rows of interambulacral plates, 

 and being consequently distinct from the newer fossil and recent 

 CidariSy I mentioned that I had long distinguished them in 

 manuscripts (in the collections at Dublin) under the name of 

 Archceocidaris. In that work I withdrew my own name however 

 in favour of Echinocrinus, by which M. Agassiz had announced 

 his intention of designating the carboniferous Cidaris Nerii, &c. 

 in his Introduction to the 2nd livr. of his ' Monog. des Echinod. 

 Fossiles,^ p. 15 : although he did not either define the genus 

 or recognise the aforesaid peculiarities, the name itself seemed 

 to indicate an entirely difi'erent affinity, namely with the Cri- 

 noidea, in which group this generic name is placed in Agassiz's 

 ' Nomenclator Zoologicus.' I propose to resume now my old 

 name for this genus, 1st, because M. Agassiz neither indicated 

 the affinities nor gave any descriptive notice of the genus Echi- 

 nocrinus, while I have done both for my Archmocidaris ; 2nd, se- 

 veral of the continental geologists have not followed my example 

 in rejecting my own name, but prefer Archaocidaris ; 3rd, in the 

 ' Catalogue Raisonne des Echinodermes,^ &c., published by MM. 



