the Morpholoiji/ and Classification of the Saleniidaj. 121 



The term sur-anal should imply a plate which is situated 

 upon the anus, and in such a sense the primary central plate 

 of young Echinidee might in its primitive position, before the 

 actual existence of an anal aperture, be styled, with some 

 reservation, a sur-anal. But to its ultimate position the terra 

 is totally inapplicable. This fact is still more conspicuous 

 in the case of the sur-anal plate of L. Agassiz in Salenia, 

 which, lying in front of the periproct, outside the ring of 

 which it forms a part, has nothing to do with the anal 

 orifice. 



As Loven has called the apical or abactinal system the 

 " dorso-central system," an appropriate term for the plate in 

 question will be the " dorso-central plate," which also has the 

 advantage of having been already employed by Herbert 

 Carpenter and other writers. 



The homology of the dorso-central plate has been the sub- 

 ject of careful study by A. Agassiz, Loven, and Herbert 

 Carpenter, and some differences of interpretation exist ; but, in 

 our opinion, the forcible arguments of the last-named natu- 

 ralist carry conviction. The case has been so clearly and 

 fully stated by him that we consider it unnecessary to recapitu- 

 late the evidence upon which his deductions are founded, and 

 that it will be held sufficient for our present purpose to state 

 that we agree with him in considering that the dorso-central 

 plate of Echinoids, Asterids, and Ophiurids is homologous 

 with the radical plate or root-disk at the extremity of the stem 

 of the Pentacrinoid larva. 



To the supplementary plates developed on the periproctal 

 membrane and surrounding the aperture of the anus the name 

 of " anal plates " has been given ; but we think that " peri- 

 proctal plates " would be a preferable term, as a means of 

 avoiding any chance of confounding them homologically with 

 the definitely-placed anal plates of the older Crinoids. 



These supplementary or periproctal plates are well seen in 

 the Cidaridffi, Saleniidse, Diadematidaj, Echinidte, and in fact 

 more or less definitely in all the Echinoidea. They show a 

 considerable amount of variability in their character and 

 disposition. In some forms they are present in the younger 

 stages as large solitary plates with more or less curved 

 outlines, which become resorbed and added to in number, 

 until a collection of various sized plates surrounds the anal 

 orifice, the largest plates being nearest the ring of the peri- 

 proct. In other forms the periproctal plates may persist as 

 four, five, or more triangular pieces having their bases at- 

 tached by ligament to the inner part of the periproctal ring, 

 their free angle surrounding and assisting in closing, when 



