154 NATUEAL HISTORY BULLETIN 



are often found in 100 to 200 fathoms. Very few species are 

 found at great depths. The Challenger Expedition took only 

 three species in depths greater than 350 fathoms. The deepest 

 was in 450 fathoms. The Albatross, in the Pacific off Panama, 

 apparently took one species, A. exiguus, in 2,136 meters (Lud- 

 wig), but there is some doubt as to the correctness of the label. It 

 also occurred in much shallower water. The West Indian species, 

 so far as known, all occur in less than 150 fathoms, and most 

 of them in very shallow water. 



The species of Astropecten are apt to be variable in many of 

 their characters, regarded as specific, and therefore their synony- 

 my is often complex. The number and size of the superomargin- 

 al spines are variable, not only with age, but independently of it. 

 Species that normally have spines may sometimes occur without 

 them. These spines are often undeveloped in the young, when 

 of considerable size, when they would appear later. The different 

 rays of the same specimen may differ as to the size, number, or 

 partial absence of spines. 



Variations in the inf eromarginal spines are also frequent, both 

 as to size and shape. All of our species except A. hraziliensis 

 normally have two to a plate, but three spines often occur on 

 some plates in certain species. None have more than three, as 

 do some foreign species. Dorsal paxillse vary considerably, in 

 some species, but in general are fairly constant. 



Perhaps the most constant specific characters are to be found 

 in the spines of the adambulacral plates, yet these vary to some 

 extent. 



Our West Indian species have three spines in the furrow-series 

 and nearly all have two unequal, usually flattened, spines in the 

 second (middle) series, wdth the aboral one larger. But a few 

 of our species have two or three slender and nearly equal ones 

 in that row. 



This is, perhaps, the most available character for dividing them 

 into two principal groups (see table below). 



The West Indian species, like most others, are mostly desti- 

 tute of pedicellarise, or have one or two rarely, but A. americanus 

 often has a nearly continuous series, of rather large size, on the 

 adambulacral plates. The same is true of some specimens re- 

 ferred to A. nitidiis v., under the varietal name forcipatus. 



