226 



Family 5— SALENIADtE. 



This natural family nearly corresponds to the Salenies of MM. Agassiz and Desor, 

 and is distinguished from other families of the Echinoidea endocyclica by the peculiar 

 structure and great development of the apical disc, which, besides the five genital and 

 five ocular plates, has an additional or sur-anal plate, developed in the centre of the disc, 

 immediately before the anal opening ; this plate is sometimes single, or more frequently 

 is composed of from three to eight separate elements. 



The test is thin, and in general small, spheroidal, hemispherical, or depressed: the 

 ambulacral areas are always narrow, straight, or flexuous, with two rows of small 

 tubercles which alternate with each other on the margins of the area. 



The poriferous zones are narrow, the pores unigeminal, except near the peristome, 

 where they fall into oblique ranks of threes. 



The inter-ambulacral areas are wide, with two rows of primary tubercles, which have 

 large bosses with crenulated summits ; in one section the tubercles are perforated, in the 

 other they are imperforated. 



The mouth opening differs in size in the different genera; the peristome is more or 

 less decagonal, and is sometimes deeply notched, or only feebly indented. The jaws 

 are known in one genus, in which they resemble those of Hemicidaris. 



The spines of one section (the Acrosalenia) are only known ; in this genus, the stems 

 are long, slender, angular, or flattened, and the surface, although apparently smooth, is 

 covered with very fine longitudinal lines. 



Erom a misconception of the true relative position of the elements of the apical disc, 

 in this family, much confusion exists in the works of different authors in the descrip- 

 tion of this part of the test. " The great difficulty in the study of this group," says 

 M. Desor,* " is to find the place of the madreporiform body ; we are consequently em- 

 barrassed when we attempt to assign the lateral parts to the longitudinal axis of these 

 animals ; unless we admit that the sur-anal replaces the madreporiform body ; but this 

 would be contrary to all analogy, because in all the other Cidarides, the madreporiform 

 body is an integral part of one of the genital plates. M. Agassiz had got rid of the 

 difficulty by means of an hypothesis, by admitting that the sur-anal plate is invariably 

 placed in the plane of the animal, that it therefore could only be anterior or posterior ; 

 hence his two divisions in the genus Salenia, — the first with a sur-anal plate posterior, and, 

 consequently, with the periprocte excentral and before ; the second with the sur-anal 

 plate anterior, and, consequently, with the j&mj»focife excentral andbehind."f 



* ' Synopsis des Ecliinides Fossiles,' p. 138. 



t For further details on this subject, M. Agassiz's 'Monographies d'Echiuodermes, premiere Mono- 

 graphie des Salenies,' may be consulted. 



