52 THE VOYAGE OF H.M.S. CHALLENGER. 



not wisli to maintaiu untenable views, and not to seem to yield to apparently overwhelming 

 evidence as to the way to use the position of the madreporic body to ascertain the axis of 

 the body. It is not my intention to argue the question anew, but I must reiterate that no 

 evidence has as yet been advanced disproving the position I start with, that the madi'cporic 

 body is placed at the suture of the embryo Echinid formed by the junction of the ends of 

 the open spu-al forming the young Sea-urchin in the Pluteus, and no amount of observation 

 on the abactinal system of young forms after they have resorbed the Pluteus, or in the 

 adult stages, will throw any light on that point. I cannot see the force of the distinction 

 made by Dr Duncan between embryonic spines and papillae, which he says cover the apical 

 disc, because they are not placed on tubercles. No young spine of any Sea-urchin is 

 articulated when it first appears, nor is it placed upon a tubercle, and I merely intended 

 to denote the fact that these appendages were embryonic spines, from their greater 

 similarity to young spines, and remained so ; and it does not seem to make a material 

 difference in the description if we call them papillae. 



From the careful description of Duncan's Salenia profundi, which is the name 

 he gives to his species, it is evident that it is more closely allied to Salenia goesiana, 

 of which it may be the adult, and that it differs very considerably from Salenia 

 varispina (A. Agassiz); but not having seen Dr Duncan's specimen I am unable to 

 associate it with Salenia hastigera, the spines of which differ radically from his. All 

 the specimens (five) collected by the Challenger in the Bay of Biscay and ofl' the coast 

 of Portugal belonged to the species which I have here called Salenia hastigera, and 

 have figured on Plate IV. of the Challenger Echinoidea. As these specimens were 

 among the first returned to Sir Wy\'ille Thomson before Dr Duncan's description 

 appeared, I am unable to examine them again and compare them with Dr Duncan's 

 description. Thomson's figures, on pp. 145 and 146, Voyage of the Challenger, Atlantic, 

 vol. i., evidently represent Salenia hastigera. This is not the only instance of 

 a Pacific species being found off Portugal. The same is the case with Aspidodiadema 

 tonsum. 



With regard to the crenulation of the primary tubercles, the primary ambulacral 

 tubercles near the astinostome are certainly, as I have stated, crenulated like the primary 

 tubercles of the interambulacral areas ; but the secondary ambulacral tubercles, which 

 continue the line of the larger primaries to the abactinal system, are not. My description 

 was not sufficiently complete, and I should perhaps have stated that the smaller 

 ambulacral tubercles are not crenulated. I did not attach special importance to this 

 character, beyond stating the fact of its existence, on account of the great difference there 

 is known to exist in other Echinidae with reference to this very point. I must also add 

 here that the lai'ge primary tubercles near the actinostome in Salenia hastigera are 

 also distinctly crenulated, but the smaller ones are not. The structure of the spines 

 most emphatically associates the Salenidae with the Cidaridae, as a cross section 



