238 On a Species of Ychinoconus. 
either side of the median line are homologous with the “ridges” 
and the other plates with the “ processes.” 
According to the inferences which arise after studying the 
British-Museum specimens, the median plate is one plate of 
a “ridge” and those on either side of it are other plates of 
the same structure. There is no ambulacral process. 
If it is admitted, as it well may be, that the specimens in 
the British Museum have had the plates on either side of the 
median suture so fused that the union is no longer visible, 
the clear definition of the ambulacral areas indicates that no 
portion of an ambulacrum exists on the flanks of the interradial 
expansions. From the evidence before us, and after studying 
Prof. Lovén’s figures, we hold that ambulacral processes or 
their homologues are absent and that the expansions are 
analogous to, and to a certain extent homologous with, the 
“ ridges ” of Cidaride. 
It must be remembered, however, that in Diadema setosum 
the “ridges” of the perignathic girdle have a median sutural 
line separating two plates, on either side of which is a plate 
clearly belonging to the interradium. The gradual evolution 
of this arrangement can be appreciated by comparing figures 
41, 42, 43, and 49 in Journ. Linn. Soc., Zool. vol. xix. 
[Dbp-co-4i5 
Finally we regret to differ from our friend respecting the 
presence of jaws and teeth in the genus Duscoidea. We 
cannot find any probable or demonstrative evidence in favour 
of their existence. 
Echinoconus (= Galerites). 
Prof. Lovén believes that a structure similar to that of the 
perignathic girdle of Déscoidea cylindrica “ maintains in 
Galerites albogalerus,” that is to say in Echinoconus. He 
also credits this well-known species with jaws and teeth. 
In the ‘ Geological Magazine,’ 1884, dec. iii. vol. i. no. 1, 
p- 10, one of us enlarged upon the nature of the peristomial 
structure of Galerites albogalerus= Hchinoconus conicus, and 
proved that the so-called teeth described by E. Forbes and 
Wright are buccal plates (p. 18) ; no jaws or teeth have been 
found. 
It was explained that no auricles have been seen in any 
specimen preserved in the British Museum, and that whilst 
the ambulacra are without processes there is thickening of the 
interradia close to the peristome. 
The whole matter has been reconsidered and with the same 
results. The five ambulacra end distinctly at the peristomial 
