AR^OSOMA LEPTALEUM. 183 



along its edges small irregularly shaped polygonal plates. The proximal 

 part of the genital plate is divided into two large plates each of which carries 

 a small secondary. The ocular plates are polygonal, separating the genitab, 

 and each carries one or two secondaries. 



The anal system is covered with irregular polygonal plates varying 

 greatly in size (PI. 75, fig. 2), with a few small elliptical plates round the 

 anal opening, arranged in two irregular rows. 



As this species is allied to A. coriaceum, it is interesting to compare the 

 details of the abactinal system given on Plate 52 of the Panamic Deep Sea 

 Echini (Mem. M. C. Z., XXXI) with those of A. eiirypatum on Plate 75 of 

 this Memoir. 



The test is scraped so nearly bare, only a very few broken, secondary 

 spines, a few triphyllous pedicellariae and some scattered sphaeridia are 

 left in a couple of abactinal folds of the test. The sphceridia (PI. 66, fig. 

 18) are remarkable for their greatly elongated form ; they are more spine- 

 like than in any other known Echinothurid. The triphyllous pedicellariae 

 are, as usual, on long, slender stalks ; the valves (PI. 66, fig. 19) are notice- 

 ably slender but expand rather abruptly at the tip, where they are very 

 flat ; they are about half a millimeter long and have a well-developed cover- 

 plate with an irregular margin. 



This remarkable specimen was taken in 1888 by the "Albatross" 

 on her way from New York to San Francisco, at the following station : 



Station 2819. Near Galapagos Islands; 6' S., 90° 6' W. Bott. temp. 

 39.9°. 671 fathoms. Wh. s. 



Arseosoma leptaleum A. Ag. and CI. 



Plates 76 and 77. 



This species belongs to the fenestratum group of Arreosoma, and may 

 be considered the Pacific representative of that Atlantic species. A single 

 specimen was collected by the "Albatross" in 1904 off Mariato Point, 

 while looking up for further investigation the green-sand patch discovered 

 by her in the Panamic region during the cruise of 1891. 



The diameter of this specimen is 125 mm. ; the greatest diameter of the 

 abactinal system 21 mm. ; of the actinal system, 30 mm. There are 19 

 plates in each half of an interambulacrum from the actinal system to the 



