78 CALAMOCEINUS DIOMED.E. 



points of the four larger plates were broken up into rectangular plates 

 with rounded corners ; so that we have in this specimen a second inner 

 ring of smaller plates immediately around the anal opening (Plate XXX. 

 Fig. 13). This arrangement of the anal plates gives to the anal system 

 much the appearance of the anal system of Bothriocidaris, in which there 

 is a ring of secondary plates within the large ring of anal plates. 



This mode of breaking up of the anal system is different from that 

 observed in the Echinidte proper, in which additional anal plates are formed 

 at one extremity of the anal system, or in that of the Cidaridge, whei'e 

 the secondary series of plates encroach more or less upon the outer ring, 

 and force their way between the plates of the primary ring. Where we 

 have five anal plates covering the anal system, as is the case in some of 

 the Arbaciadce, these five plates occupy a radial position, as in Arbacia 

 Dufresnii, though this is not the case in the specimens of Arbacia nigra 

 with five anal plates. 



In Podocidaris prionigera (Plate XXX. Fig. 11) there are five anal plates, 

 but they are not radially placed. In the other species of Podocidaris there 

 are only four plates, as in Podocidaris sculpta (Fig. 10), and as is usually 

 the case in Arbacia and Coelopleurus (Fig. 6). In one specimen of Coelo- 

 pleurus floridanus (Fig. 7) there were indications of the formation of eight 

 anal plates by the splitting radially of the four original plates. The Arba- 

 ciadae with only four anal plates we may consider as having one of the 

 five radial plates atrophied. Bell mentions one specimen of Arbacia in 

 which he found only three anal plates. 



If we are to regard the existence of an anal ring of five, and subsequently 

 of ten, anal plates as the oldest type of anal plating among the Echini, 

 what we have seen taking place among the Cidarida? and Arbaciadae in 

 the way of a reduction and variation in the number of plates of the outer 

 anal ring may assist us in finding a true explanation for the apparently 

 .anomalous condition of things in the Salenidte and in the young stages 

 of many Echinidse proper. 



De Loriol has given a figure of the apical system of Acrosalenia, the 

 precursor of Salenia, in which the greater part of the anal plates are 

 preserved, and which, as has been noticed by previous authors, shows 

 some affinity to that of the allied Cidaridae (Plate XXX. Fig. 4). This 

 interesting vspecimen seems to me to give the key to the apparently ab- 

 normal condition of the Salenidas. In the specimen figured by de Loriol 



