494 ECHINUS MICROTUBERCULATTS. 



The primary spines are sharper, more slender, shorter, and are much more 

 closely striated than in acutus, of a greenish tinge, usually slightly darker 

 at the base. The abactinal system closely resembles that of acutus. 

 The genital openings are not so near the edge of the plates, and there are 

 from five to seven small tubercles near the anal edge, with the rest of genital 

 plates frequently well covered by miliaries, while in acutus there are coarser 

 secondaries and a much smaller number of miliaries. The median vertical 

 sutures of the ambulaeral and interambulacral space form a plainly marked 

 zigzag line, usually double, diminishing in breadth towards either pole. 



Mediterranean. 



Echinus microtuberculatus 



! Echinus microtuberculatus Blaixv., 1825, Diet. Sc. Nat. O. 



PI. VI. f. 4; PI. VIP./. 1-2; PL VIII. f. 19-21. 



A great deal of the confusion concerning the identity of the Mediterranean 

 species arises from the fact that their northern representatives are found 

 associated with them ; as. for instance, E. miliaria and E. microtuberculatus, 

 E. acutus and E. melo, E. cordatus and E. mediterraneum ; and, also, that 

 sometimes where no difference exists, — B. lyrifer = B. pulvinatus; C. 

 papillata = C. hystrix, — the same species frequently appears under a 

 different specific name if found in the Mediterranean from that it receives 

 when coming from the Atlantic and the Northern Ocean. In many of the 

 museum collections I have examined, E. miliaris and E. microtuberculatus 

 are not distinguished when the localities are not marked otherwise than 

 as Mediterranean. Yet the two species are remarkably distinct ; it is not 

 the young of E. melo, as has been suggested, but a Psammechinus in the 

 old sense, with strongly imbricated buccal membrane. The actinal opening 

 is very small, circular, with the slightest possible cuts. The poriferous zone 

 is narrow, the pores being arranged almost vertically, with but a slight 

 obliquity, forming three pairs. The test is thin, flattened ; the abactinal 

 system large, prominent, with a very large anal system, covered by small 

 plates of uniform size, slightly tuberculated : the genital ring is comparatively 

 narrow ; genital and ocular plates covered by small secondary tubercles, 

 nearly of uniform size ; madreporic body but slightly larger than other geni- 

 tal plates; ocular plates excluded from the anal system ; genital oj^enings 

 large. Two principal vertical rows of large tubercles in the ambulaeral and 

 interambulacral space ; rest of coronal plates filled with closely arranged 



