72 CALAMOCRINUS DIOMED.E. 



anal plate of the Salenia?, which I considered to have retained in the adult 

 stages a feature only found in the embryos of the Echinida? proper. 



From a re-examination of the subject, I am now more inclined to look upon 

 the plates of the young of the EchinidiB, and the suranal plate of Salenia, not 

 as an embryonic feature, but as the remnant of the anal plates which hfive 

 gradually disappeared from the anal system through such genera as Acrosa- 

 lenia and Salenia. We unfortunately know but too little of the plates of the 

 anal system of the fossil genera, which, connected as they are by a thin mem- 

 brane, can naturally only be preserved under very exceptional circumstances. 



Taking Bothriocidaris as an example of the oldest Sea-urchin known of 

 which the plates of the anal system are in part preserved, we find five 

 radials and five interradials. forming a single ring round the anal system. 

 At the angles of the radial plates five small anal plates are situated, sepa- 

 rated from one another by a smaller intercalated plate, thus forming an 

 outer anal ring of ten plates, the inner part of tlie anal system being evi- 

 dently covered by still smaller plates (Plate XXIX, Fig. 1). 



In geologically younger Sea-urchins, the Palaechinidte, we have still a 

 single ring of radials and interradials enclosing the anal system, which has, 

 as in Bothriocidaris, an outer ring of ten anal plates and a second interior 

 ring of smaller plates, as in Palaechinus elegans (Plate XXIX. Fig. 2). 



While it is true, as Neumayer says, that the apical system of the Palae- 

 chinidae, as far as the adult is concerned, presents no trace of the anal 

 plate so prominent in 3'oung Echinoids, and which is permanent in Salenia, 

 yet there is nothing to show that the young of those genera did not possess, 

 like the Euechinoidea, a single central anal plate in their earliest stages. 

 So that it seems as if the anal system of the Arbaciadoe, with their four 

 anal plates, was perhaps derived from that of Cidaridaj and Palsechinidae. 

 There are a limited number of anal plates (eight) in Aspidodiadema Jacobyi 

 (Plate IX." Figs. 4, 13).* 



I have often been puzzled how to interpret the many pores found on 

 the genital and ocular plates of the Palajchinidne, such as are figured by 

 Roemer and by Meek and Worthen. On carefully reading the description 

 given by Schmidt t of the apical system of Bothriocidaris, t it becomes 



* Report on the Echini of the " Blake," Mem. Mus. Corap. Zobl., Vol. X. No. 1, 188.3, by A. Agassiz. 



t Mem. Acad. Imp. St. Petersb., XXI., No. 11, 1874, p. 39. 



t The structure of the test of Bothriocidaris is represented by the buccal system of Cidaris. One 

 range of interambulacral plates extending from the apex to the actinostome intercalated between rows 

 of ambulacra with two pairs of pores each. May not the double rows of later Palfechinidee have 

 ari.sen by the opening out and the separation of the superposed plates? 



