OCR Text |
Show 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. franciscanus. Turning from S. albus, the presence of which on the Chilian coast was signalized by Molinal, 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 measurements of the largest of the three specimens :- Diameter of T * ^ actino- abactinal anal Poriferous Ambul. Interamb. Length. Height, stonie. system, system. zone. area. area. 108 49 25 20 1P5 7"5 24 40 It will be seen that the test is not high, that, although the arcs pores are so nearly horizontal in direction, the ambulacral areae 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. franciscanus; and the scrobicular circle around which they form a ring in the interambulacral, though not in the ambulacral areae, 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. lividus, the dentary apparatus is not as much as one half the height of the test; but the fenestrae (or spaces between the alveoli) are proportionally much shorter, and the radii are long enough to reach to the margin. Three specimens were sent:- (1) Trinidad Channel, shallow water. (2) T o m Bay. (3) Cockle Cove. STRONGYLOCENTROTUS, sp. inc.2 (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 1 Molina, ' Saggio sulfa Storia nat. del Chile,' Bologna, 1782, p. 200. 2 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 the arc of pores may vary very considerably during growth. I give a full description and figure of it, to exhibit the marked differences which obtain between it and the adult. |