1881.] THE SURVEY OF H.M.S. 'ALERT.' 89 



arcs of pores ; but these tubercles are much smaller and much more 

 irregular in S. bullatus, and this new species has also a somewhat 

 larger mouth (actinostome), though it by no means has one so large 

 as has S. /ranciscanus> Turning from S.. albus, the presence of 

 which on the Chilian coast was signalized by Molina ', to S. gibbosus, 

 the other member of the genus which has been hitherto recorded 

 from this district, we find in it only four pairs of pores in each arc, 

 while the much smaller test has a proportionally larger actinostome. 

 The following are the more important measuremeuts of the largest 

 of the three specimens : — 



Diameter of 



actino- abactinal aual Poriferous Ambul. luteramb. 

 Length. Height, stoine. system, system. zone. area. area. 



108 49 25 20 11-5 To 24 40 



It will be seen that the test is not high, that, although the arcs of 

 pores are so nearly horizontal in direction, the ambulacral arese are 

 not very wide; they are, indeed, only provided with two rows of 

 primary tubercles, which are never very large, and, like those of the 

 interambulacral series, decrease to quite a small size on the greater 

 part of the actinal surface. The number of secondary tubercles (or, 

 rather, of large miliaries) is very much greater than in S.francis- 

 canns ; and the scrobicular circle around which they form a ring in 

 the interambulacral, though not in the ambulacral arese, is not so 

 wide as in that species. There is no petaloid enlargement of the 

 poriferous zone around the actinostome ; the number of pores in an 

 arc may be here and there reduced to seven. The large number of 

 small tubercles on the plates of the abactinal system is very striking, 

 as is, too, the large size of the anal and madreporic plates. The 

 auricles are well developed, and the space between the two halves 

 elongated and triangular. As in S. lividits, the dentary apparatus is 

 not as much as one half the height of the test ; but the fenestrse 

 (or spaces between the alveoli) are proportionally much shorter, 

 aud the radii are long enough to reach to the margin. 



Three specimens were sent : — 



(1) Trinidad Channel, shallow water. 



(2) Tom Bay. 



(3) Cockle Cove. 



Strongylocentrotus, sp. inc. '^ (Plate VIII. figs. 3, 4.) 



This specimen is at once distinguished by the very remarkable 

 arrangement of the arcs of pores, which are so little bent as to be 

 better indeed called rows, and are, above the ambitus, set very 



^ Molina, ' Saggio sulla Storia nat. del Chile,' Bologna, 1782, p. 20(). 



^ I was for a long time inclined to regard this specimen as a representative 

 of a new species ; but a long and close study of other members of the genus has 

 convinced me that the form of tlie arc of pores may vary very considerably 

 during growtii. 1 give a full description and 6gure of it, to exhibit the marked 

 differences which obtain between it and the adult. 



