SEA-URCHINS. 



167 



In the family Aebaciidjs the median interambulacral spaces show as so many- 

 bare bands, and the structure of the jaws, teeth, auricles, and spines, is intermediate 

 between the Cidaridae and Echinidas. The species are few. A. punctulata occurs 

 upon the eastern, and A. nigra upon the western coast of this country. In Coelopleurus 

 the spines of the primary tubercles are immense, three times as long as the diameter 

 of the test, and taper gradually to a fine 

 point. One species occurs on the coast 

 of Florida. 



The Salenidjj: are a small tribe with 

 spines like those of the Cidaridse in struc- 

 ture; and with the anal and genital 

 plates soldered together. Salenia vari- 

 spina is quite common in the Caribbean 

 Sea at depths from three hundred and 

 fifty to one thousand six hundred and 

 seventy-five fathoms. This species has 

 a small purple body and long white ser- 

 rated spines, and in appearance resem- 

 bles Dorocidaris. The character which 

 removes it into another family seems a 

 very small one, yet is one in which it 

 diffei-s fi-om all regular sea-urchins, ex- 

 cept its own immediate relatives, which, 

 so far as we know, commenced to live 

 upon this earth in Jurassic times, and 

 have continued through cretaceous and 

 tertiary to the present day. Instead 

 of having five ocular and five genital 

 plates in its rosette, this little urchin has 

 eleven, the additional one large, cres- 

 cent-shaped, and occupying a central 

 position. This plate thrusts the anus 

 quite out of the centre of the rosette. 



In the DiADEMATiD^ the spines are 

 hollow, long, and set with rings or ver- 

 ticillations. The test is thin, and the 

 spines delicate, so that it is very diffi- 

 cult to preserve a specimen entire. D. 

 mexicanus occurs, on the west coast of 

 Mexico, while D. setosum is found in 

 both oceans. In Echinothrix the test is stouter than in Diadema, and there are many 

 vertical rows of very small tubercles instead of the larger tubercles of uniform size 

 which characterize Diadema and Astropyga. E. desorii of the Pacific and Red Sea 

 attains a diameter of about five inches, while the spines do not exceed half the 

 diameter of the test, and are often banded with greenish yellow. 



In Astropyga the test is so thin as to be more or less flexible, and is greatly de- 

 pressed, the height usually not exceeding one-third, or even one-fourth of the diameter. 

 In life the colors are very bright, the ambulacral plates have pits or depressions of a 



Fia. 146. — Salenia varispina, enlarged. 



