MOIRA CLOTHO. 217 



It is interesting to trace the gradual specialization of the odd anterior 

 ambulacrum from the time of the Chalk ; Holaster, Hemipneustes, Cardi- 

 aster, Toxaster, all have lateral ambulacra flush with the test or nearly 

 so, features still retained among the recent Spatangoids in Aerope and 

 Aceste ; while in Hemiaster, Periaster, Prenaster, and Schizaster, in addi- 

 tion to the specialized odd ambulacrum, the lateral ambulacra become 

 somewhat sunken, and culminate in such sunken lateral ambulacra as 

 those of Abatus and some species of Schizaster, and finally in Moiropsis 

 and Moira, in which all the ambulacra are sunken. These genera have 

 been preceded by such types as Linthia with ambulacra all equally but 

 slightly sunken within a peripetalous fasciole, while in Brissus, Meoma, 

 Metalia, and the like we have stages of development representing the 

 growth of the sunken lateral ambulacra in Abatiis cavernosus} In Moira 

 we find the greatest development in the sunken ambulacra, not only of 

 the lateral pairs but also of the odd anterior ambulacral pouch, which is 

 more sunken ^ than that of the lateral pairs (PI. 109, figs. 2, 5). 



Moira dotho differs from M. atropos in having the abactinal system more 

 excentric, and in being more elliptical in outline. In M. clotJio the fasciole 

 passing under the anal system crosses the plates in contact with the anal 

 system, while in M. atropos it cuts across the posterior extremity of the set 

 of plates next to the sternum. In M. atropos the posterior extremity of the 

 sternum at the ambitus is much narrower than in M. dotho. In M. dotho 

 the anal system is cut out of five interambulacral plates ; in M. atropos from 

 four only. 



Station 2800. Lat. 8° 51' N. ; Long. 79° 31' 30" W. 7 fathoms. Bottom 

 temperature, 77°. Gn. m. 



Station 2801. Lat. 8° 38' N. ; Long. 79° 29' 30" W. 14 fathoms. 

 Bottom temperature, 78°. Gn. m. 



Both Stations in the Bay of Panama (Voyage of the " Albatross " from 

 New York to San Francisco, 1887-1888). 



Bathymetrical range, 7-14 fathoms. Temperature range, 78°-77°. 



1 "Hassler" Echini, PI. IV, figs. 4-8. 



2 Revision of the Echini, PI. XXIII, figs. 3, 5. 



