142 Papers from the Marine Biological Laboratory at Tortugas. 



come in at this exact period of growth could only be known from obser- 

 vations on a larger series of specimens than was available. In living 

 specimens, opened when perfectly fresh, it was found that the teeth extend 

 only very slightly above the base of the foramen magnum. As this struc- 

 ture is very shallow in Eucidaris, the dorsal border of the tooth very 

 nearly coincides with the upper limits of the lantern, but the tooth does 

 not extend horizontally over the top of the lantern as in Strongylocentrotus 

 (Phylogeny of the Echini, p. 179, plate 5, figs, i, 6). The dental capsules, 

 which are the fleshy bags inclosing the base of the teeth, are small in Euci- 

 daris, not large and inflated as they are in Strongylocentrotus and in the 

 Echinidae. 



The Cidaridae, the sole family representing the order Cidaroida, is the 

 only group of modern Echini that extends back into the Palaeozoic. The 

 Palaeozoic species, which belong to the genus Miocidaris, present only 

 slight differences from recent typical members of the order. As an ancient 

 group that has sulTered comparatively little change in its whole geological 

 history the structure and development of the Cidaridae is of especial inter- 

 est. Something is known of the post-embryonic development of cidarids, 

 due principally to Loven's critical study of young Goniocidaris (Echinologica, 

 Stockholm, 1892), but further knowledge of the developing stages of this 

 primitive archaic type is much to be desired. 



Centrechinus setosus (Leske;. 

 The most abundant sea-urchin at Montego Bay is Centrechinus'^ setosus 

 (Leske), which abounds in countless profusion in shallow water on the 

 reefs. In strong distinction from the quiescent habits of Eucidaris, Cen- 

 trechinus is a most active animal, moving about freely and actively and 

 waving its long spines in a threatening fashion. The spines are so sharp 

 and cause such poisonous wounds that they are a constant menace to 

 bathers, and specimens have to be collected with caution or serious results 

 will follow. The tube-feet have only the slightest hold on the sea-floor, so 

 that specimens can be picked up with long-handled forceps without any 

 perceptible resistance. In this Centrechinus differs markedly from Trip- 

 neustes, Strongylocentrotus, or Echinometra, which on a rocky surface cling 

 tenaciously by the tube-feet. Young specimens of Centrechinus have spines 

 which are strongly banded; white and very dark, nearly black, alternating. 

 The light and dark bands are of about equal width. The spines are long 

 in young as well as in adults, and a specimen 12 mm. in diameter has spines 

 36 mm. in length. A specimen 60 mm. in diameter has spines up to 150 mm. 

 in length. From this it occurs that a small specimen looks rather big with 

 spines extending radially, and a large specimen is menacing indeed when the 

 needle-like sharpness of these weapons is considered. Banded spines as a 



> Centrechinus is a name which I gave (Phylogeny of the Echini, p. 27) to replace the name Diadema, 

 which in post-Linnaean usage is preoccupied for a crustacean. To replace an old, established name is unfor- 

 tunate, and some objection has been raised to my action. As Diadema has been in current use as a generic name 

 for sea-urchins, Gastropoda, and Lepidoptera, it seems that the rule of priority must be maintained and a new 

 generic name substituted for Diadema in these three groups. 



