2 PROCEEDINGS OF THE NATIONAL MUSEUM vol. m 



arose from an ancestral stock that was radially symmetrical. This 

 topic is discussed briefly elsewhere (Pawson, 1966) and it is hoped 

 that a full discussion will arise from research now in progress. 



Gross features of the internal anatomy of psoUds are similar to 

 those in other dendrochirotid holothm-ians; such modifications as are 

 present can be explained readily in terms of the altered shape of the 

 body. The calcareous ring, for example, lies inside the mouth as 

 usual, but, as the mouth is turned dorsally, the calcareous ring is 

 oriented in the anteroposterior plane, so that the posterior edge of 

 the ring is now ventral. Tube feet are very numerous on the ventral 

 surface of the body and restricted to the radii, forming a continuous 

 fringe around the sole. The midventral radius is usually naked in 

 Psolus but carries a varying number of feet in other members of the 

 family. Dorsal tube feet are reduced (as in Psolidium, Stolinus, 

 Lissothuria) or absent altogether (as in Psolus). 



Within the family Psohdae the genus Psolus seems to be the most 

 highly modified. Other genera, especially Psolidium and Lissothuria, 

 show more features in common with other dendrochu-otids, and within 

 the genus Psolidium it is difiicult to determme whether or not some 

 species should be regarded as members of the Psolidae; they might 

 be referred equally weU to one or another family elsewhere in the 

 Order Dendrochirotida. 



An attempt is being made to revise the entire family Psohdae. The 

 present paper comprises a revision of the small genus Lissothuria, 

 which is restricted to the eastern Pacific Ocean and the Caribbean 

 Sea region. 



I would like to thank Dr. WiUard D. Hartman of the Peabody 

 Museum, Yale University, for many kindnesses, Drs. Bent Hansen 

 and F. Jensenius Madsen of the Zoological Museum, Copehagen, for 

 their help during my visit to that museum in the summer of 1965, 

 and Captain Fred C. Ziesenhenne of the Allan Hancock Foundation, 

 Los Angeles, for facihtating my study of the collections at that 

 institution. 



Status of the Genus Thyonepsolus Clark 



H. L. Clark (1901a, p. 167) described a new holothiu-ian from the 

 coast of California. The species differed from all other psolids 

 known to Clark at that time in possessing calcareous deposits of a 

 unique type on the dorsal surface of the body. In addition to the 

 overlapping scales characteristic of psolids, the dorsal surface carried 

 hourglass-shaped and tower-shaped calcareous deposits. In view of 

 the natiu-e of these deposits, Clark proposed a new genus, Thyonepsolus 

 nutriens, to accomodate this species. 



