NO. 5 CLARK : ECHINI OF WARMER EASTERN PACIFIC 261 



Lytechinus semituberculatus (Valenciennes) 

 Plate 41, Fig. 16 



Echinus (Psammechinus) semituberculatus L, Agassiz and Desor, 1846, 



p. 368. 

 Lytechinus semituberculatus VtrnW, 1867, p. 301. 



Mortensen, 1943, pp. 456-459, pi. 26, figs. 

 5,6. 



This well marked species is represented in the Velero collection by 863 

 specimens, but it should be stated at once that 570 of them are bare tests 

 from a sand beach adjacent to lava rock at Bindloe Island, Galapagos 

 Islands. The largest specimen at hand is 43 mm in diameter but most of 

 them are considerably less than that. The yellow green color of the 

 primary spines is very characteristic of well grown specimens, but very 

 3^oung individuals, 5 mm or less in diameter are bright vermilion red or 

 paler, even a simple cream color with the median interambulacra apically 

 deep green. Individuals 8-15 mm h. d. are often very handsome with the 

 test deep green above and more or less pure white beneath ; the dorsal half 

 of each interambulacrum may be deep red in sharp contrast with the ver- 

 milion red and white of the ambulacra. The apical S5^stem in such speci- 

 mens is greenish with a few vermilion red spots. In some young individuals 

 the red may be very pale except orally, and in others it may be wholly 

 wanting, the resulting green and white livery being soon replaced by the 

 brownish test and long green spines of the adult. In youth the green shades 

 are rich and rather dark and more or less mixed with white but with in- 

 creasing size the white as well as the red tends to disappear. It is hard to 

 believe that the really beautiful red, green, and white j'oung urchins can 

 so soon and so completely lose their beauty and become the uniformly dull 

 brown or blackish adult covered with uniformly yellow-green spines. 

 Some adults have the spines tinged at the tip with yellow brown and 

 rarely the color of a dried specimen may be definitely brown with little 

 indication of green. In the great majority of museum specimens, however, 

 the characteristic yellow green or light green shade of the primaries is 

 unmistakable. 



Distribution. — This urchin is one of the most typical of Galapagos 

 echinoderms. The Velero took it at 43 stations in the archipelago ; 10 times 

 near Charles Island, 8 near James, 6 at Albemarle, 5 at Hood, 4 at Inde- 

 fatigable, 3 near South Seymour, 3 at Barrington and one each at Bar- 

 tholomew, Bindloe and Tower. Not a specimen has been secured at Cocos 



