280 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 8 



ambulacral plate carries a similar but slightly larger spine at each end. 

 Miliary spines are fairly numerous among these primary and secondary 

 spines so the test is normally very completely but not densely covered. The 

 abactinal system of plates is similarly well covered but the peristome is 

 quite free from spines. Many, perhaps most adult specimens differ from 

 this typical spinulation in one of two ways. On the one hand the primary 

 spines are disproportionately bigger, 15 mm or more in length and more 

 than a millimeter thick basally. In extreme cases the primaries are over 20 

 mm long and 2 mm thick at base. On the other hand, the primary spines 

 increase markedly in number but decrease in thickness and length ; they are 

 6 or 7 mm long and scarcely more than half a millimeter in thickness. This 

 diminution of the primaries occurs for the most part above the ambitus 

 and may continue to such an extent that there are no normal primaries on 

 the upper half of the test, but the whole abactinal surface is covered by a 

 fairly dense coat of small secondaiy and miliaiy spines. Whether this 

 condition is due to some feature of the environment such as constant heavy 

 surf or to a diseased condition resulting from parasites or some deficiency 

 of the environment, we do not know, but the result is a sea-urchin whose 

 identity would be difficult to determine without a good series of connecting 

 links. 



Aside from environmental conditions modifying structure and appear- 

 ance, there is no doubt that hybridization with franciscanus is a frequent 

 if not a constant factor in the life history of purpuratus. As the two species 

 often occupy the same regions and even the same environmental areas, it is 

 not strange that cross fertilization often occurs and as a result many indi- 

 viduals are found whose correct identification is too much for even an 

 experienced taxonomist. Color is the most useful factor in determining the 

 species for typical purpuratus ; it is at first green and then becomes purple 

 and when large enough to be called adult it is practically entirely purple 

 as the name indicates. The larger species, franciscanus, is never purple nor 

 does it have any trace of such purple as characterizes purpuratus. Young 

 individuals are light brown, almost or quite a fawn brown, and usually 

 this becomes increasingly dark, until very large adults are a very dark 

 violet brown, sometimes almost if not quite black. But there is never any 

 indication of the purple of purpuratus. Hybrids usually show some indi- 

 cation of purple on the primary spines. A second indication of hj'bridi- 

 zation is seen in the stoutness of the primar}^ spines. While very large 

 individuals of purpuratus may have unusually large primary spines they 

 are never equal to the normal spines of franciscanus. Oftentimes specimens 

 occur which have the general form and appearance of purpuratus but the 



