ARBACIADAE. 399 



ARBACIADAE. 



Family Arbaciadae Gray, 1855, Proc. Zool. Soe. London. 



The Echini composing this family are few in number ; but the characteristic 

 features are so striking, as intermediate between the Goniocidaridae and the 

 Diadematidae and Echinidae proper, that it was impossible to include them 

 in either of the above families. The absence of the characteristic secondary, 

 miliary, and granular tubercles of the Echinidae, the limitation to the num- 

 ber of vertical rows of primary tubercles, much as in Diadematidae ; the 

 peculiar structure of the actinostome, — are features which separate this small 

 group from either of the families mentioned. The structure of the jaws is 

 peculiar : they are elongate, as in Cidaris, with a small foramen ; the arch of 

 upper extremity of jaw is open, as in Cidaridae and Diadematidae. The teeth 

 are shaped as in the Diadematidae and Echinidae proper. The auricles are 

 disconnected. The poriferous zone is simple ; pores not arranged in hori- 

 zontal rows, somewhat as in Goniocidaridae. The abactinal system is, in all 

 genera thus far found to belong to this family, peculiarly ornamented. The 

 anal system consists of only four large triangular plates. The structure of 

 the spines is intermediate between that of the Goniocidaridae and Echinidae. 

 The centre of the spine, as well as the exterior, is made up of small lime- 

 stone cells, as in Cidaris, closely packed, irregularly arranged, with the excep- 

 tion of a ring at some distance from centre, made up of large, open limestone 

 cells, as in Echinidae. The outer sheath of the spines of the Arbaciadae is 

 limited to the extremity of the spines, forming a sort of cap. This has 

 been well described by Desmoulins. 



ARBACIA. 



Arbacia Gray, 1835, Proc. Zool. Soc. London. 

 (See Part II. p. 263.) 



Arbacia Dufresnii 



! Echinus Dufresnii Br.., 1825, Diet. Se. Nat. O. 



! Arbacia Dufresnii Gray, 1835, Proc. Zobl. Soc. London. 



The dry denuded tests of this species, frequently occurring in collections, 

 are at once distinguished from any other species of the genus by the brilliant 

 green bare band in median interambulacral space, extending nearly to the 

 ambitus. This bare band is flanked by only one row of large primary 

 tubercles, distant, occurring frequently only on every other coronal plate ; 

 the intermediate tubercles being small, only two ; the inner row composed 



