330 PROCEEDINGS OF THE ACADEMY OF [1890. 



The regular arrangement of the j)hites produce? a superficial re- 

 semblance to an Asterias. 



The specimen collected has a diameter of about 50 mm. R:==5r. 



This species has as yet only been recorded from Vera Cruz. It 

 differs essentially from Echhutster as defined by Perrier,' Viguier' 

 and Sladen,'' and even as defined by Miiller and Troschel* them- 

 selves, for they say, " in der Haut ein zusammenhangendes netz 

 von Balken," and quadrilateral plates certainly cannot be termed 

 " Balken." The abactinal and lateral skeleton of an Ech'master 

 consists of small, narrow, imbricated plates forming an irregular 

 network. The skeleton in this species, on the contrary, consists 

 mainly of quadrilateral plates arranged in regular longitudinal 

 series. It is also distinguished by the minute tuberculation of the 

 plates, the arrangement of the spines along their edges and the 

 thorny character of the distal portion of these spines. I have, there- 

 fore, erected a new genus for this species, which may be character- 

 ized as follows: — 



THYRASTEK, n. gen. 



Ambulacral pedicles biserial. Actinostomial margin defined by 

 the adambulacral plates. Arms subcylindrical, skeleton composed 

 mainly of quadrilateral plates, united at their angles by imbrication 

 and arranged in longitudinal series. Plates minutely tuberculated, 

 supporting spines upon their edges. Distal portion of the spines 

 microscopically thorny. 



I hope at a future period to consider the question of the relation- 

 ship of Thyraster to the Echinasteridre in general. 



Astropecten articulatus, Say, Journ. Acad. Nat. Sci. Phila. (1) Vol. 5. ji. 141. 

 Perrier, Arch. Zool. Exp., Vol. 5, 1876, p. 290. 



Astropecten duplicatus (xray, Ann. Mag. Nat. Hist. Vol. VI, 1841, p. IS], 

 Perrier, op. cit. p. 2V1. 



Astropecten Valenciennii, Muller and Troschel. System der Asteriden. p. ()8. 

 Astropecten variabilis, Lutken, Vidensk. Meddel., 1859, p. 59. 



A single specimen of this species was collected at Vera Cruz. 



It is found in the West Indies, extending northward to Beaufort, 

 N. C. and southward off the island of Jolbo on the northeast coast 

 of Yucatan. 



1 Arch. Zool. Exp. t. iv, pp. 283 and 289. 



2 Ibid. t. vii, p. 123. 



^ Challenger Report, pp. xxxviii and 53B. 

 * System der Asteriden, p. 22. 



