NO. 3 DEICHMANN : HOLOTHURIOIDEA ; PART I, DENDROCHIROTA 101 



One specimen, which was opened, had few but well-developed genital 

 tubes. The color of the 4 individuals is light brown with darker freckles; 

 the general impression of the preserved material is a nondescript grayish- 

 brown color. 



Genus 7. THYONACTA, new genus 



Diagnosis. — Medium-sized to large form, 10-15 cm. long in con- 

 tracted condition. Skin thick, filled with spicules. Tube feet in 5 bands 

 and in varying number in the interambulacra, depending on the animal's 

 age ; toward the oral and anal ends thickened valves are developed. Many 

 tube feet are conical, papilliform. Tentacles 10, the 2 ventral smaller. 

 Calcareous ring low but with short posterior projections; stone canal 

 small, attached to the dorsal mesentery; Polian vesicle single, ventral in 

 position ; retractors exceptionally short, attached closely behind the cal- 

 careous ring (when the crown of tentacles is withdrawn). Gonads 

 placed unusually far back, in the posterior third of the body. 



Spicules consist of an external layer of simple baskets, and an inner 

 layer of 4-holed buttons of different size, also a few larger plates ; mostly 

 regularly knobbed. Feet apparently without any end plate (at least in 

 larger individuals) ; walls with numerous supporting rods and plates 

 besides buttons and baskets. Introvert with rosettes ; tentacles with large, 

 mostly narrow perforated plates and rods which decrease in size toward 

 the tips of the branches. 



Type species. — Thyonacta sabanillensis (Deichmann). 



Remarks. — ^At present 2 species have been referred to this genus, both 

 restricted to the American waters. Very likely some of the members of 

 the old genus Thyone from the western Pacific and Indian oceans ought 

 to be transferred to this genus. The type species came from the West 

 Indian seas, while the other species, described below, has been secured 

 from various localities in the Panamic region. 



Thyonacta mexicana, new species 

 Plate 17, Figs. 1-13 



Diagnosis. — As for the genus. Spicules rather stout and deep baskets 

 and buttons of 2 types, a large and strongly knobbed one and a smaller, 

 less regularly knobbed and often with accessory holes. Color varying 

 shades of brown. 



