Miscellaneous, 239 



a modified form of Pseudodiadema, and has probably arisen from the 

 adoption of a deep-sea life resulting in diminished calcification of 

 the test. 



2. ' On Echinocystis and Palceodiscus, two Silurian Genera of 

 Echinoidea.' By J. W. Gregory, D.Sc., F.G.S. 



The Author gives a history of the genera Echinocystis, Salter, 

 and Palceodiscus, Wyv. Thorns., redescribes their structures, and 

 discusses their affinities. He concludes that Echinocystis is an 

 echinid and not a cystid ; and that Palceodiscus is an echinid and 

 not an asterid. In order to prevent confusion, he suggests the 

 name Scolocystis for a true cystid which was described by Hall as 

 Echinocystis, some years after the latter name had been applied by 

 Wyville Thomson to the fcssil now concluded to be an echinid ; and 

 also suggests the name of Discocystis for the form named Echino- 

 discus by Worthen and Miiller. 



In discussing the affinities of Echinocystis, Wyv. Thorns., the two 

 latest diagnoses of the Cystoidea (those of Prof, von Zittel and Prof. 

 Haeckel) are considered, and it is contended that they do not enable 

 us to draw any sharp line of distinction between cystids and 

 echinids. 



It is shown that the masticatory apparatus of Palceodiscus and 

 Echinocystis explain the origin of that structure in gnathostomate 

 echinids ; and furthermore, it is suggested that Echinocystis renders 

 probable the homology of the so-called 4 calycinal plates' of the 

 Echinoidea with the plates of the valvular pyramid of the Cystoidea, 

 and not with the calyx-plates. 



The Author gives synopses of the order Cystocidaroidea of Prof, 

 von Zittel, and of its two families, Palaeodiscidse and Echinocystidae. 



MISCELLANEOUS. 



To the Editors of the 'Annals and Magazine of Natural History.' 



The Generic Name of the River Crayfish. 



GENTLEMEN, It is unnecessary for me to reply to the Rev. Mr. 

 Stebbing at any length. 



(i.) Reference to vol. xi. of the Trans. Linn. Soc. will confirm the 

 accuracy of my remark as to Leach's treatment of Astacus and 

 Nephrops. I am surprised at Mr. Stebbing supposing that I cited 

 an anonymous article of uncertain date. 



(ii.) Mr. Stebbing appears to be unaware of rule 2 of the rules 

 proposed by the British Association for zoological nomenclature. 

 What is there said of Brisson applies literally to Gronovius, and 

 the genera of the one author are as good as those of the other. 



(iii.) Does not Mr. Stebbing know that 1758 has been well called 

 by the most eminent of the exponents of the zoological works of 



