HEMIASTER EXPERGITUS. 165 



Hemiaster. 



Agassiz and Desor, 1847. Ann. Sci. Nat. Zool., (3), 8, p. 16. 

 Type, Spatangus bufo Al. Brongniart, 1882. Cuvier's Oss. Fgjs., 2, p. 320, 604. 



This is a very ill-defined group of spatangoids. A large number of Fossil 

 species have been referred to it and its exact limits are still uncertain. Some 

 nine Recent species have also been called Hemiaster. The first of these to be 

 described was Lov6n's H. expergitus and I see no reason to doubt that this 

 is really congeneric with H. bufo. Mortensen has shown that H. mentzii and 

 H. gibbosus of A. Agassiz are identical with expergitus, and that Mr. Agassiz's 

 H. zonatus is in part expergitus and in part an unidentifiable Brisaster. Studer's 

 H. florigerus is not a Hemiaster; so far as I can see, it is a Paraster and I have 

 included it in that genus. On the other hand Mr. Agassiz's Periaster tenuis 

 is a Hemiaster, as the genus is here restricted. No doubt many of the Fossil 

 species referred to Hemiaster have no right in the genus in this restricted sense. 



Key to the Species of Hemiaster. 



Petaloid area small, so peripetalous fasciole is not near ambitus. 



Sternum short and wide, its width .65-.85 of its length; test high, v. d. at posterior 

 end .65-.90 length. 



Height of test .65-75 length; peristome about .20-.25 of test-length from 



anterior end expergitus. 



Height of test about .90 length; peristome about .30-.32 of test-length from 



anterior end globulus. 



Sternum longer and narrower, its width only about half its length or less. 



Test very high posteriorly, v. d. about .80-.90 length vanus. 



Test more flattened, v. d. only .60-.65 length tenuis. 



Petaloid area large and very conspicuous so peripetalous fasciole is close to ambitus . . hickmaiii. 



Hemiaster expergitus. 



Lov6n, 1871. Ofv. Vet. Akad. Forh., no. 8, p. 6, fig. (desc. impar). 1874. fitudes sur les fich., p. 13. 



I have nothing to add to Mortensen 's extensive account of this species 

 (1907. Ingolf Ech., pt. 2, p. 97-103). The large series in the Albatross collec- 

 tion confirms his opinion that gibbosus cannot be distinguished from expergitus, 

 and there is no advantage in attempting to maintain it, as he suggests, as a 

 "variety" merely because it occurs in the western Pacific instead of the Atlantic. 



Station 2751. West of Montserrat, B. W. I. Bott, temp. 40°. GS7 fins. 

 Bu. glob. oz. 



