136 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.8 



intestine is attached in the left ventral interambulacrum, while in the 

 last genus it is attached in the right interambulacrum. This may indicate 

 that the Thyonepsolus-Psolidiurn series represents an independent branch 

 and the Psolus series another, or it may simply be that the mesenterial 

 attachment has shifted completely over in the most advanced genus. Some 

 of the members of the genus Psolidium have very few dorsal feet, and 

 these have formerly partly been referred to the genus Psolus. 



Key to the Genera of the Family Psolidae Known from the 



Panamic Region 



1. Dorsal side (including oral and anal ends) without appendages 

 (except the anal papillae and in some forms a few abortive feet 

 on the introvert) 3. Psolus Oken p. 146 



1. Dorsal side with a varying number of tube feet, usually perfor- 

 ating the scales 2 



2. Dorsal side characterized by the presence of large hourglass- 

 shaped bodies. Tube feet on dorsal side with walls stiffened by 

 numerous curved squarish or circular plates, sometimes reduced 

 to crosses (also a few oblong supporting plates may be pres- 

 ent) ; towerlike deposits present in most species, often reduced 

 with age 1. Thyonepsolus H. L. Clark p. 136 



2. Dorsal side without large hourglass-shaped bodies, at utmost 

 small cups or baskets. Dorsal tube feet in some forms supported 

 by squarish plates, in others by oblong plates or rods, in others 

 superficial deposits seem to be entirely lacking on the dorsum; 

 towerlike deposits sometimes present. 2. Psolidium Ludwig p. 141 



Genus 1. THYONEPSOLUS H. L. Clark, 1901 



Thyonepsolus H. L. Clark, 1901, p. 169.— Deichmann, 1930, p. 192; 



1937, p. 172. 

 Psolidium Ludwig, 1904, p. 689. 

 Lissothuria Verrill, 1867, p. 322. 



Diagnosis. — Small to medium-sized forms with 10 tentacles, the 2 

 ventral smaller; sole sharply set off; feet numerous in 3 crowded bands. 

 Dorsal scales few to numerous, covering the back more or less completely ; 

 feet numerous or few; a well-developed external layer of spicules covers 

 the scales. 



