CLARK: EXTENDED RANGE OF GENUS LYDIASTER 141 



ZOOLOGY. — A new starfish (Lydiaster americanus) from the 

 Gulf of Mexico, representing a section of the subfamily Go- 

 niasterinae hitherto knoivn only from the Indo-Pacific region. 1 

 Austin H. Clark, National Museum. 



One of the most interesting of the new starfishes described in 

 1909 by Professor Koehler from the collections of the Royal 

 Indian Marine Survey Ship Investigator was Lydiaster johannae 

 which, with the closely allied genus Circeaster decribed at the 

 same time, represented a hitherto unknown type of Gonias- 

 terinae. Lydiaster johannae was dredged only in a single locality 

 off the southwestern coast of Ceylon in 401 fathoms, while the two 

 species of Circeaster were found off the western coast of Ceylon 

 and southern India in from 912 to 1053 fathoms. Thus, so far 

 as known up to the present time, these two genera are confined 

 to the Arabian Sea. The only genus closely related to these two, 

 Mariaster, occurs off southern Japan. 



But it appears that Lydiaster also inhabits the Gulf of Mexico, 

 being represented in this region by a remarkable new species, in 

 some respects intermediate between Lydiaster and Circeaster. 

 This may be known as 



Lydiaster americanus, sp. no v. 



Five arms; R = 100 mm.; r = 35 mm.; R:r = 2.9: 1; superomar- 

 gi rials 24. 



General form pentagonal, with relatively narrow, evenly tapering- 

 arms, the length of which, measured actinally along the curve from a 

 point directly below the commencement of the enlarged plates, is equal 

 to the distance from the same point to the center of the interbrachial 

 arc opposite. The diameter of the arms at the base (at the level of the 

 distal border of the fifth supermarginal) is 19 mm. In the outer half 

 the arms bend gradually upward, so that their terminal portion makes 

 an angle of about 60 degrees with the plane of the disk. 



Compared with L. johannae the interbrachial arcs are more nearly 

 straight and the arms, which are narrower, appear to arise more 

 abruptly. 



The abactinal surface, which is somewhat swollen, is covered with 

 small polygonal plates which are mostly subequal in size and irregular 

 in arrangement; they vary from about 1 mm. to about 2 mm. in diame- 

 ter, being usually about 1.5 mm.; just before the base of the arms the 



Published with the permission of the Secretary of the Smithsonian Insti- 

 tution. 



