62 ECHINOIDEA. I. 



biscriatum, has been referred to this especially well characterized genus; but it has not hithertho been 

 more thoroughly described, so that for the present nothing can be said of this species. 



One more genus will have to be established for a large Echinothurid obtained by the Ingolf >- 

 Expedition. The tube feet form one irregular series on the actinal side; the spicules irregular fene- 

 strated plates not arranged in series; no sucking disk. The primary spines on the actinal side curved, 

 with large hoof. Three kinds of pedicellariae are found: tridentate, ophicephalous, and triphyllous 

 pedicellarise. The tridentate ones occur in two forms; in the larger form (length of the head up to 

 3'5 mm ) the blade is filled by a coarse, very thorny net of meshes (PI. XII. Fig. 41). The edges are not 

 involuted; the outer part of the blade somewhat widened. The neck very short, the stalk of the 

 common structure. The smaller form resembles those in PJi. placenta, but the widenings from the 

 upper end of the apophysis reach to the edge of the blade. The ophicephalous pedicellariae (PI. XIV. 

 Figs. 19, 23, 25) are very peculiar, the upper end of the valve being widened in a wing-shaped way, 

 while the middle part is very narrow. The length of the head ca. 0-5". The neck is quite short, 

 contrary to the ophicephalous pedicellariae of the Echinids, and the stalk is a thick, perforated tube. 

 As ophicephalous pedicellarise , as far as hitherto known, are not found in other Echinothurids 

 (perhaps they are found, however, in Hygrosoma luculentum (see above p. 59 60), but then they have 

 quite another form) they yield an excellent character for this genus. In the triphyllous pedicellariae 

 the cover-plate is rather slightly developed, richly perforated (PI. XII. Fig. 31). For this genus I pro- 

 pose the name of Tromikosoma 1 ). 



According to these researches the system of the Echinothurids gets the following appearance: 



Phormosoma Wyv. Thomson (emend.). 



The primary spines on the actinal side straight, club-shaped, inclosed by a thick bag of skin; 

 marked difference between the actinal and the abactinal sides. The areoles of the actinal side very 

 large. The tube feet are arranged in a single series on the actiual side. The spicules large fenestrated 

 plates; no sucking disk. Only tridentate and triphylloiis pedicellariae. The tridentate ones are simply 

 leaf-shaped, with little developed net of meshes. The widenings from the upper end of the apophysis 

 do not reach to the edge of the blade. The stalk of the pedicellariae irregularly perforated. 



Species: Ph. placenta Wyv. Thomson, bursarium A. Ag., rigidum A. Ag. 



Distribution: Northern part of the Atlantic, Japan, the Philippines, New-Zealand. -- Archiben- 

 thal forms. 



Echinosoma Pomel (emend.). 



The primary spines on the actinal side curved, with a little hoof at the point; the actiual and 

 the abactinal sides look almost quite alike, only a few, large spines being found near the ambitus. The 

 areoles large. The tube feet are placed in one almost regular series on the actinal side; the spicules 

 large fenestrated plates, no sucking disk. Only tridentate and triphyllous pedicellariae. Of tridentate 

 pedicellariae two forms are (always?) found, a large one, flat, with a rich net of meshes, and with the 

 upper end of the apophysis continuing some way into the blade as a serrate crest, and a smaller one, 



') rpofttxog quivering. 



