REPORT ON THE ECHINOIDEA. 71 



Station 188. September 10, 1874. Lat. 9° 59' S., long. 139° 42' E.; 28 fathoms; mud. 

 Honolulu; 18 fathoms. July 30, 1874. 



ECHINOTHURID^. 



Family Echinothuridze, "Wj'ville Thomson, Depths of the Sea, 1873, p. 164. 



WyviUe Thomson, 1874, Echinoidea of the "Porcupine," Trans. Roy. Soc, vol. clxiv. part 2, p. 730. 



The characteristic overlapping of the ambulacral coronal plates has been well 

 described and figured by Thomson (Echinoidea of the " Porcupine," Trans. Eoy. Soc, vol. 

 clxiv. part 2, p. 730, pi. Ixv. and following ; see also A. Agassiz, Eevision of the Echini, 

 pi. ii.''). I have here given some additional details mainly regarding Phormosoma , and 

 have also called attention to the changes in the family characters due to growth. 



Thomson speaks, in the Depths of the Sea, of the vermicular movements passing- 

 through the test of Asthenosoma, when it assumed on deck what appeared to be 

 its normal form and attitude. When handled, the test moved and shrank from the 

 touch, and had much the feeling of the disk of a Solaster or other large Starfish. I 

 have in one of my letters to the Superintendent of the U. S. Coast Survey ' spoken of 

 the globular form of the species of Asthenosoma when they came up in the trawl, but I 

 can only corroborate the statements of Thomson regarding the peculiar movements of 

 the test of specimens when on deck, due undoubtedly to the great mobility of the plates 

 of the test. It is quite dangerous to handle these specimens when alive (or even in 

 spirits) ; the wounds they made with their numerous minute sharp stinging spines pro- 

 ducing a decidedly unpleasant sensation, accompanied with a slight numbness, fully as 

 painful as that occasioned by the stinging of a Physcdia. 



Some of the species of Echinothuridse here described, show that some of the important 

 characters upon which this family is distinguished from the Diadematidse may become 

 gradually obliterated, and the existence of such species as Phormosoma asterias, Astheno- 

 soma gracile, and Phormosoma 7^igidum, where the lapping of the plates is reduced to a 

 minimum, if it exists at all, and where, as in all the young of the group, the distinction 

 between the actinal and abactinal surfaces, so striking in the larger sj)ecimens, does not 

 exist, and develops only with increasing size, show how difficult it is to separate this 

 group of Echinids as a distinct family from the Diadematidse. 



The lapping of the coronal plates in the Echinothuridae is not so absolutely a charac- 

 teristic feature of the famUy as has been supposed. It exists already well-developed in 

 Astropyga (PI. X." fig. 9), but with this important difi"erence, that the overlapping of 

 the plates is in the same direction in both areas. The lower edge of the plate passes 

 under the upper edge of the preceding plate. In Echinothr-ix and Diadema also, the shape 



1 Letter No. 3, BuU. Mus. Comp. Zool., vol. v., 1879. 



