128 ROBERT TRACY JACKSON ON ECHINI. 



siderable range is shown, however, and as all exsert is the typical feature, variants are therefore 

 all progressive. In 2 % ocular I is insert, in 5 % V, in 25 % I, V, and in 2 % I, V, IV are insert. 

 Aberrant variants are frequent, 13%. Of these, one specimen has ocular III alone insert, a 

 variant seen in only two other sea-urchins, four specimens have V, III insert, a unique variation, 

 two have V, IV, III insert, which is the dominant character of Strongylocentrotus gibbosus 

 (text-fig. 156, p. 145), and one specimen has oculars I, V, IV, III insert. It is striking that so 

 many and such peculiar aberrant variants occur in this species, and the same is true of other 

 species from the west coast of South America as noted (table, p. 164). Mr. Agassiz (1873, p. 

 438) says of this species that all oculars except one are excluded from the periproct, but I find 

 that there is much variation and that all oculars exsert is the specific character. 



Strongylocentrotus fragilis sp. nov. 



This new species is striking for its size and the extreme thinness of the test. In shape it 

 is strongly depressed, almost flat ventrally, the ambitus almost coinciding with the base. The 

 color is reddish claret; young specimens are orange red; spines are short, 10 to 20 mm. long, light 

 colored. Ambulacral plates have five elements each. Interambulacral plates ventrally have 

 a row of primary tubercles up to six in number to each plate. At and above the ambitus there 

 is a vertical series of one prominent primary tubercle to a plate in both interambulacral and 

 ambulacral areas. Secondary and miliary tubercles are very small. Pedicellariae are similar to 

 those of S. drobachiensis. In the type the diameter of the apical disc through III, 5 is 18 mm.; 

 of the peristome in the same plane 27 mm. The test is very thin and correspondingly fragile, 

 0.6 mm. thick at the mid-zone. Of seven specimens in my collection all are of about the same 

 size, but that selected as the type (R. T. J. Coll., 838) measures 94 mm. in diameter at the 

 ambitus and 44 mm. in height. The specimens were collected at Catalina Island, off the south- 

 ern California coast, and were sent me by Ward's Natural Science Establishment at Rochester, 

 New York. A number of specimens are in the collection of the United States National Museum, 

 dredged off the coast of southern California and Oregon in depths of 124 to 339 fathoms. These 

 are smaller, about half grown, and are included in the tabulation of ocular plates. Of this 

 species (55 specimens) 56% have oculars I, V insert as the typical character. Of arrested 

 variants 13 % have all the oculars exsert, 29 % have I only insert, and 2 % have V only insert. 

 There are no progressive variants or aberrants. Photographic figures, in dorsal and side view 

 of the holotype of this species will appear in Part 4 of Agassiz and Clark's Hawaiian and 

 other Pacific Ephini, Memoirs of the Museum of Comparative Zoology, Vol.34; Plate 113, 

 figs. 3, 4. 



Of Strongylocentrotus mexicanus only six specimens were seen; five have oculars I, V and 

 one has V only insert. In Strongylocentrotus tuberculatus (39 specimens) 92 % have oculars I, 



