66 ECHINOIDEA. I. 



all the genera established here as good ones, and also the limitation of the old genera PJiormosoma 

 and Asthenosoma is no donbt correct. Only the genera Arceosoma and Hygrosoma are perhaps still 

 taken in too wide a sense, in as far as the species A. tesselatum and Belli, as also H. luculentum 

 onght perhaps to be separated as particular genera; at all events, however, they are most nearly allied 

 to the genera to which they are here referred. 



In stead of the former confusion of species and the two genera that were not to be kept 

 distinct, we have got a number of definitely characterized and easily recognisable genera — a result 

 that has been obtained especially by a careful examination of the pedicellarise. Thus it proves here 

 as in the Cidarids to be a fact that the spines and the structure of the test are in no way a sufficient 

 basis for the classification. Otherwise the spines play a prominent part in the classification of the 

 Echinothurids, and by means of these alone a far better classification might have been obtained than 

 the one expressed in the old < genera Phormoso)iia and Asthenosoma. 



For the present it must be left undecided whether there may be any question of a grouping 

 of the genera into subfamilies. There is, however, no doubt that the genera Phormosoma and Kamp- 

 tosoma are rather distantly allied to the other genera. 



5. Phormosoma placenta Wyv. Thomson. 



PI. IV, Figs. 1-2. PI. XI, Figs. 7, 10, 25. PI. XII, Figs. 2—3, 7, 11, 19, 21, 23, 25, 26, 37, 39. PI. XIII, Fig. 7. 



Synonym: Phormosoma Sigsbci Agassiz. 



Principal literature: Wyville Thomson: Porcupine -Echiuoidea (395). p. 732. PI. LXII — LXIII. 

 — A. Agassiz: 6. p. 75. Blake-Echini (9) p. 30. PI. XII, XV. Fig. 3 — 19. — E. A. Verrill: 418. p. 139. 

 - W. E. Hoyle: Rev. List of Brit. Ech. (202). p. 406. -- F. Jeffr. Bell: 69. p. 436—38. Catalogue of 

 Brit. Ech. (73). p. 144. 



This species has been so carefully described by Wyv. Thomson and Agassiz, that there is 

 no reason to give here again a complete description of it. Only a few structures need still a more 

 exact description, viz. the spines, the tube feet, and the pedicellarice ; some remarks must also be made 

 with regard to the development and transformation of the apical area, as also with regard to the inner 

 structure. 



Of the spines on the actinal side of this species Bell(Catal. p. 144) says: «from what is known 

 ... it is probable, that they are rather long and have a stout calcareous cap . This is wrong. Wyv. 

 Thomson, to be sure, says (I.e.) that two kinds of spines are found, but what he describes and figures 

 is only larger and smaller spines of the kind found on the abactinal side; the large spines on the 

 actinal side have been broken in his specimens. Agassiz, in the description of Pli. placenta (<: Blake - 

 Echini), says nothing of the spines of the actinal side, but from his fig. 8. PI. XII it is seen that they 

 are club-shaped, and in the explanation of the figures they are called <clubshaped . In the diagnosis 

 of Ph. Sigsbei, which, according to Agassiz himself, is synonymous with Ph. placenta, it is said: 

 primary radioles on the actinal surface resembling those of Ph. 6/trsar/a-, and of these latter he says 

 (Chall. Ech. p. 100): :on the actinal surface the primary spines are not tipped with a solid hoof, but 

 all end in a fleshy bag . — Thus it may be seen, by comparing the several statements, to be sub- 



