Prof. Agassiz on the Echinodermata. 193 



monograph which it is my intention shortly to pubUsh upon 

 the hving species of the genus Echinus (Auct. Antiq.) I have 

 estabhshed the following cli\"isions, of which I here only in- 

 dicate the typical species : Tenmopleurus (Ech. toreumaticus), 

 Pleurochinus (E. bothyroides), Microcrjphus (E. versicolor), 

 Trijmeustes (E. ventricosus), Amblypneustes (E. griseus), Tax- 

 opneustes (E. pileolus), Stomopneustes (E. variolaris). I have 

 also thought it better to form three subdivisions of the genus 

 Arbacia, restricting that name to the small fossil species, and 

 proposing that of Trayyp)us for the species whose anus is 

 closed by a valve of four pieces, and that of Agarites for those 

 which have the interambulacral areas partially destitute of 

 tubercles and spines. 



The Holotlmrioi are always the dinsion of Echinodermata in 

 Avhich there remains the most to be effected. The difficulty of 

 observing these animals has hitherto not enabled the science 

 to be supplied with designs {j)lanches) sufficient for its exi- 

 gencies. There are none but those which are pubhshed by 

 MM. Quoy and Gaimard in the ' Zoological Atlas of the As- 

 trolabe,^ and which, as it regards their execution, do not leave 

 anything to be wished for ; but as it respects their distribu- 

 tion into genera, a great deal too much of vagueness and un- 

 certainty is presented by the characters selected as the basis 

 of the groups. 



Okeu (Lehrbuch der Naturgeschichte, 1815) was the first 

 to divide the Holothnrm into four genera, which he calls Thy- 

 one, Subucu/us, Holothuria and Psolus. Lamarck only ad- 

 mits two genera in this family, the Holothuria and the Fistu- 

 Jarice ; these genera again are not well determined. Cuvier, 

 without giving generic names to his divisions, has nevertheless 

 established, in the first edition of the * Regne Animal,' six very 

 natural sections in the genus Holothuria, the first of Avhich 

 corresponds to the genus Psolus of Oken, the second to tlije 

 genus Cuvieria of Peron, the third and fourth to the true Ho- 

 lothurice, the fifth and sixth to the genus Subuculus of Oken, 

 which is also synonymous with the genus Cucumaria, Auct., 

 or Pentacta of Goldfuss ; lastly, the sixth corresponds to the 

 genus Thyone of Oken, which Mr. Fleming has named MUl- 

 leria in his ' History of British Animals,' but which is not the 

 genus Mulleria of Jeeger. Eschscholtz subsequently esta- 

 blished in the ' Zoological Atlas,' after the second voyage of 

 Capt. Kotzebue round the world, two new genera under the 

 names Synapta and Chisodota, which correspond to the genus 

 Tiedemunnia of Leuckardt (Isis, 1S31, Comptc rendu de la 

 reunion des Naturalistes Allcmands a Hambourg). M. Leuck- 

 cudt has also estabhshed the genera Phascolosoma and Oche- 



