SALENID^. 145 



sometimes deeply notched, or only feebly indented. The jaws are known in one genus, 

 in which they resemble those of Hemicidaris. 



The spines in Acrosalenia are long, slender, angular, or flattened, and the surface of 

 the stem, although apparently smooth, is covered with very fine longitudinal lines. In 

 Pseudosalenia they are enlarged, ovoid, snb-glandiform, and more or less granular; in 

 PeUastes and Salenia they are elongate, aciculate, straight, curved, or bent. 



From a misconception of the true relative position of the elements of the apical disc in 

 this family, much confusion exists in the works of diffei-eut authors in the description 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 embarrassed 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 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 periprocte 

 excentral and behind."^ 



Professor Johannes Midler assigns the left posterior genital plate as the bearer of the 

 tiiadreporiform body in Saletiia persona/a. " Dies wird auch durch die Salenien bestatigt, 

 wo die Langsachse durch die plaque suranale vor dem After bestimmt wird. An einem 

 im mineralogischen Museum aufbewahrten ausgezeichnet schonen Exemplar der Salenia 

 personafa. Ag., mit vorderem After, Taf. I, fig. 9, ist die linke hintere Genitalplatte poros 

 und Madreporenplatte." ^ 



I have selected fine specimens of PeUastes Wriglitii, Desor, from the Lower Green- 

 sand, PeUastes Austeni, Forbes, from the Lower Chalk, and Salenia petalifera, Desniarest, 

 from the Upper Greensand, and in all of these the madreporiform body occupies the 

 surface of the right anterior genital plate, as in the Cidaridce, Hemicidarida, Biademadce, 

 and EcJiinida. The sur-anal plate is central, and the anal opening posterior in a line with 

 the axis of the body, or inclined to the right side. In fact, the madreporiform body and 

 sand canal, whatever their true functions may be, have the same position in all the 

 Echinidea, recent and fossil, which I have examined, and probably the same in all 

 Echinodermata. Professor Midler's mistake, therefore, may have arisen from jjlacing the 

 Salenia in a false position before him. The study of the apical disc in the Acrosalenia 

 reveals the true relation of its elements to each other, and jjroves that the sur-anal plate 



1 ' Synopsis des i5chiiiiiJes fossiles,' p. 138. 



2 For further details on this subject, M. Agassiz's 'Monographies d'Echinodermes: premiere Monographie 

 des Salenies,' may be consulted. 



3 Job. Miiller, ' Ueber den Ban der Echinodernien,' p. 7. 



19 



