TIIK AMin.A< KIM oi TIIK r<w<>.\\ til 



A similar condition of fan-shaped plates proximally, which an- m.t - -h.-ip-il di-tally. i- Men 

 in Melonechinus mulliporus (Plate 50, figs. 4, 5) and M. i/ii/milm* (Plate HI, CIRH. 5-9). 

 Comparable differences between the interior and exterior side- of tin- -ame plates are BW-II in 

 interambulacral, ocular, and genital plates, us docribed in tin- consideration of these purl- pp. 

 75, 96, 172). All this shows that in the study of specimens and the description uf -peeie-, ,,i,,- 

 must bear in mind whether the specimen represents the internal or external characters of I lie te-t. 



Another feature noticed on the inside of ambulacra! plate- i- elevated nodo-e or -pino-e 

 projections that occur in some Echini. These were first seen in the fo-.-il Il\atterhinu> 'Plate 

 24, fig. 6), where low, knob-like or spinose elevations exi.-l between the inner [tore and tin- 

 middle of the area. Alexander Agassiz (1904, p. HI) has described spine- extending into tin- 

 body from the inner face of the peristornal ambulacral plates of I'ororiiltiris mlinsi, but I In-lieve 

 they have not been noticed before in coronal ambulacral plates. In Phyllacanthu- Plate :{. 

 fig. 12) near to the ventral border of the test, there are small spines between the inner pore.- 

 and the middle of the area; these seem to be the equivalent of those occurring in the Palaeo/nie 

 Hyattechinus. Close to the peristornal border these spine< increase in height, arch over and 

 then fuse in a continuous ridge (text-fig. 224, p. 193). Passing dorsally, in Phyllaeanthii- a 

 second series of spines occurs, one over each inner pore, and above the ventral area the-e 

 alone exist. In Eucidaris tribuloides I find a similar condition to that of Phyllacanthu-. except 

 that there are commonly three spinules over the inner pore (Plate 3, fig. 13). Such >pino-- 

 growths are most delicate and are destroyed by a touch so that they can only be -een in a te-t 

 cleaned without any brushing. They have not been noticed in any of the (Yntrechinoida. 



Closely associated with the ambulacrum are the spheridia described by I .oven (1874), 

 which, as he showed, are characteristic of all modern Echini except the C'idaridae. If they had 

 existed in pits, as is so often the case, we might expect to find them in the Palaeozoic. The 

 fact of their absence in the primitive Cidaridae is an argument for the aumption that they 

 were also probably absent in Palaeozoic genera. Certainly they are not known. 



Pedicellariae have recently come into prominence through the critical -tudie.- of Morten-en 

 and others. It is of interest to note that tridentate pedicellariae occurred in the Palaeozoic, 

 as shown in Meekechinus elegans gen. et sp. nov. (Plate 76, figs. 8, 9). Except a- >lmwn in 

 the Jurassic Pelanechinus by Groom (1887) I believe these are the first found fossil, and they 

 will probably always remain rarities. 



Special respiratory organs are also closely associated with the ambulacra. ( 'harle- Stewart 

 (1879) first described internal branchiae in Eucidaris tribuloidr* which lie dorsal to the lantern 

 and extend over the ambulacral areas radially from beneath the compa-e- and between the-e 

 and the braces. The same structures were described by Ludwig (1880) and by Pn.uho 1 1887) 

 in Cidaris papillata. Prouho gave them the name of Stewart's organs, a convenient distinction 

 from the quite different external gills. I have seen them in dissections of Culnrin ajfnw, 



