134 Prof. P. M. Duncan and Mr. W. P. Sladen on 



it is equally visible in Salenia petalifera. The other fossil 

 species do not appear to present the crowding necessary to pro- 

 duce the deformation. The sphteridia discovered by one of us 

 some years since, and the recognition of the external branchi^ 

 in the same form, Salenia 'profundi. Dune, 1877 (Ann. & 

 Mag. Nat. Hist. ser. 4, vol. xx. p. 245), were strong points 

 against associating Salenia with the Cidaridge, and now the 

 presence of a perignathic girdle with ridges and processes 

 decides the necessity for the separation, which has received 

 the sanction of A. Agassiz. 



It is well known that the dorso-central plate of the Salenice 

 has an angle pointing to the suture between the basals 2 and 

 3, and that the periproctal ring is not posterior, but is formed 

 at the expense of the right posterior corner of the dorso- 

 central and of the' inner edges of the basals 1 and 5. The 

 dorso-central is in contact with basals 2, 3, 4, and in slight 

 contact with basals 1 and 5. This is seen in S. scutigera 

 and in the Eocene S. Blanfordi, also in S. varispina, in which 

 the basals 5 and 1 are just touched, and this is also the case 

 in S. Pattersoni, A. Agass. S. hastigera has the same arrange- 

 ment. 



The question of the orientation of a Salenia, Peltastes, or 

 Acrosalenia need not quite depend upon the appreciation of 

 the position of the madreporite. In a species which has given 

 A. Agassiz and one of us some trouble, or rather a specimen 

 of a species which Agassiz has since perfectly defined, viz. 

 ;S'. varispina, there were two basals perforated by the madre- 

 poric body. The result of placing the madreporite seen by 

 one observer as a body in the right anterior basal (no. 2) was 

 to throw the anus posterior to the dorso-central plate, and to 

 suggest that the form was more Peltastic than Salenian. But 

 A. Agassiz, selecting the other basal (no. 1), which was also 

 perforated, came to the inevitable conclusion that the species 

 had the anus excentric behind and to the right hand, that is to 

 say it was a Salenia *. It is not uncommon to find the genital 

 duct opening so large and ragged in more than one basal in 

 fossil forms that the simulation of the ragged opening of the 

 madreporite is perfect. Under the circumstances the opjDor- 

 tunities of making mistakes are at hand, and it may be im- 

 possible to settle which basal is no. 2, so as to place radial III. 

 in front, and proceed to determine the antero-posterior axis 

 of the apical system. 



Often no trace of a madreporite can be seen. Under such 

 difficulties the method of Loven of distinguishing the oblique 



* Revision, pi. iii. figs. 9 & 11 ; also Report on ' Blake ' Echini. 



