96 THE VOYAGE OF H.M.S. CHALLENGER. 



figured the teeth of another type of Phorniosoma {Phormosoma teniie) on PL XVIII.'', 

 these teeth differ consideraljly in the size of the foramen of the pyramids, and the shape 

 and prominence of the median ridge. The jaws of Phormosoma temie are proportionally 

 more solid than those of Phormosoyim placenta, and also broader in proportion to their 

 height. 



The plates of the actinal membrane imbricate, as do those of the test, away from the 

 actinostome. In the abactinal system the membrane is only partly covered by the plates, 

 leaving a marked reticulation between them ; the tubercles of these plates are very 

 indistinctly crenulate and surrounded by a broad smooth areolar space. The whole 

 surface of the abactinal part of the test is dotted by delicate pigment (violet) spots. In 

 the ambulacral areas these spots are closely packed round the openings for the passage of 

 the suckers, forming a delicate ring at the base ; the suckers near the abactinal region 

 become pointed (PL XVIII." fig. 12). Suckers are present on the actinal region 

 somewhat beyond the ambitus (PL XVIII." fig. 13). On the abactinal surface of 

 a specimen measuring 168 mm. in diameter (PL XVIII. fig. 7), the primary tubercles are 

 arranged on the interambulacral area in two irregular vertical rows. Towards the 

 ambitus the tubercles are more numerous, arranged in thi-ee or four irregular rows. The 

 secondary tubercles are irregularly arranged, increasing in number towards the edge of 

 the test. In the ambulacral area there are two irregular vertical rows of primary tubercles, 

 and, adjoining the amlxilacral pores, on the interior edge, secondary tubercles are arranged 

 in an irregular vertical row ; these, as well as the primary tubercles, become more numerous 

 towards the edge of the test. The primary tubercles of the abactinal region are smaller 

 than the large primary tul^ercles which cover the actinal region, being really intermediate 

 in size between them and the secondary tubercles. The pedicellaria3 of the abactinal 

 surface of the test are scattered over the whole surface of the test. They are, however, 

 more numerous in the amljulacral area, and in the adjoining part of the interambulacral 

 area. The pedicellarias are long stemmed with a small head articulating with a second 

 stem, from twice to three times the length of the head (PL XVIII.'' fig. 11). A second 

 kind of pedicellaria with an inverted conical head, and a comparatively stouter joint 

 articulating upon a long stem occurs not unfi'equently upon this surface of the test (PL 

 XVIII.'' fig. 13). They resemble those figured by Thomson for Calveria hystrix. 



Close to the ambitus on the actinal side the primary tubercles take a great develop- 

 ment, the scrobicular areas of the tubercles of the interambidacral and the single one of 

 the ambulacral area occupying the greater part of a plate, but except on three or four of 

 these large primary plates the other tubercles near the actinostome soon assume the 

 proportions of those of the abactinal surface (PL XIX.* fig. 2), but they are placed close 

 together. 



The secondaries, miliaries, and pedicellarise, are also far more numerous on the actinal 

 side of the test, and we find there a third kind of pedicellaria with a shorter articulation 



