the Morphology and Glass ijicat ion of the S;ileiiii(U«. 119 



central disk, far from having" an ephemeral existence as in the 

 Echinidffi, is permanent, occurring througliout the lifetime of 

 an individual as a persistent and solid plate, growing with the 

 other pieces of the skeleton, and filling the central s])ace with 

 its lamina, which is perfectly pentagonal. When the anal 

 orifice opens out, it becomes partially eroded at the edge by 

 resorption." He concludes : — " But the central disk always 

 retains, in a perfectly recognizable manner, its primitive pen- 

 tagonal form, and it is evident that it is not a sur-anal or sup- 

 plementary plate occurring in Salenia and its allies, but is a 

 normal part of the skeleton which persists throughout the 

 life of the animal." 



Herbert Carpenter, following A. Agassiz and Lov^n, 

 homologizes the sur-anal })late of Salenia with the primary 

 central plate of the abactinal system of Echinus ; and further- 

 more, in his instructive essay " On the Oral and Apical 

 Systems of Echinoderras " (Quart. Journ. Micr. Sci. n. s. 

 vol. xviii.), he establishes, on logical grounds, the tx-ue homo- 

 logy of a corresponding plate iu the Crinoidea, Asteroidea, 

 and Ophiuroidea. 



Alex. Agassiz, with his ciiaracteristic generosity, sent to 

 one of us a very fine and large specimen of Salenia Patter- 

 sont, A. Ag., in spirit ; and its study has enabled us 

 thoroughly to appreciate our friend's admirable descriptions 

 of the species in the " Report on the Echini of the ' Blake ' 

 Expedition," p. 13, pis. iv. and v. 



In this specimen the sur-anal plate is large and as well 

 defined as in any fossil Salenia ; it is, as is usual in Saleniw, 

 incomplete in the right posterior angle, and this part is, as 

 usual, eroded more or less by the periproct. The plate forms 

 a part of the ring of the periproct. The periproct is large, and 

 has a rather stout membrane, which is attached to the edge 

 of the ring formed by the sur-anal and the basals 1 and 5 ; 

 the anal orifice is central and the space between it and the 

 ring is covered with well-formed plates, which are large near 

 the ring and smaller and more numerous near the orifice, and. 

 most carry stunted spines. (See also A. Agassiz, ' Blake ' 

 Echini, pi. iv. fig. 18.) 



1'hese plates of the periproctal membrane, which become 

 more numerous with age, are the homologues of the similarly 

 placed plates of Echinus, Strongylocentrotus, &c., and they 

 bear the same relation to the orifice of the anus within the 

 periproctal ring. 



Alex. Agassiz examined the young forms of Salenia dredged 

 by the ' Blake ' with the expectation that they might " throw 

 some light on the formation of the sur-anal plate and its homo- 



