62 ECHINOIDEA. I. 



biscriattivi, 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 b\- the Ingolf »- 

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

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

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

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

 3-5'"") 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 Ph. placenta^ but the widenings from the 

 upper end of the apophysis reach to the edge of the blade. The ophicephalous pedicellariæ (PL 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 pedicellariæ of the Kchinids, and the stalk is a thick, perforated tube. 



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



(perhaps they are found, however, in Hygrosojiia liiculcnfuiii (see above p. 59 — 60), but then they have 

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

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

 pose the name of Tromikosoma'). 



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 actinal side. The spicules large fenestrated 

 piates; no sucking disk. Only tridentate and triphyllous pedicellariæ. The tridentate ones are simply 

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

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



Species: Ph. placenta Wyv. Thomson, btirsarium A. Ag., rigidiuii 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 actinal 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 piates, no sucking disk. Only tridentate and triphyllous pedicellariæ. Of tridentate 

 pedicellariæ two forms are (always?) found, a large one, flat, with a ricli 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, 



') TpOfJLtxiig — quivering. 



