142 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 8 



age). Dorsal tube feet with or without end plate and supporting rods or 

 plates in the walls ( the type species has squarish plates of similar type as 

 those in Thyonepsolus). 



Type species. — Psolidium dorsipes Ludwig. 



Remarks. — The genus comprises a number of species. A few of these 

 have formerly been referred to Psolus because the few dorsal tube feet 

 were overlooked. On the other hand, it seems as if Psolidium convergens 

 Herouard (1901, p. XXX) ought to be transferred to some other genus, 

 judging from the description given by Perrier ( 1905, p. 38, pi. 2, figs. 2-4, 

 text figures D-F). 



The genus is closely related to Thyonepsolus H. L. Clark; actually 

 Ludwig considered (1904) the latter as a synonym of Psolidium. The 

 similarities between the 2 genera are even greater than assumed since it 

 has been discovered that the curved plates in the type species are partly 

 supporting plates for the dorsal tube feet and likewise that the character- 

 istic towers also may be present (see below). Nevertheless, it seems justi- 

 fiable for the present to retain the 2 genera on account of the more com- 

 plex hourglass-shaped bodies which are typical in ThyonepsoluSj while 

 Psolidium has small cups or baskets. 



Four species are discussed here, all represented in the Allan Hancock 

 collections and all taken at moderate depth or even in shallow water. One 

 is the type species, while 3 are described as new. 



Key to the Species of Psolidium Known from the Panamic 

 Region and Adjacent Waters 



1. Tube feet fairly numerous in the midventral ambulacrum, also 

 numerous on the dorsal side where the scales are small and nu- 

 merous. Sole with plates with fairly large holes, smooth or 

 knobbed, and deeply convex cups. Dorsal side with numerous 

 deep cups and often also towerlike deposits; tube feet with 

 numerous curved squarish supporting plates, a few are narrow, 

 rod shaped 1. Psolidium dorsipes Ludwig 



1. Tube feet in midventral ambulacrum few. Dorsal feet 

 easily overlooked. 2 



2. Dorsal scales with a varying number of prominent pearllike 

 projections. Spicules in sole 4-holed buttons or plates with 

 knobbed surface and varying from delicate to stout. Oral 

 opening with large triangular scales; many small scales inter- 



