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



at which the specimens were dredged. Specimens of the same species from shallower 

 regions, having pretty generally a comparatively stouter test. I may refer to the 

 evidence furnished from the different species of Pourtalesia, Cystechinus, and Urechinus. 



Station 133. October 11, 1873. Lat. 35° 41' S., long. 20° 55' W.; 1900 fathoms; 

 bottom temperature, 1 -3° C. ; globigerina ooze. 



Station 205. November 13, 1874. Lat. 16° 42' N., long. 119° 22' E.; 1050 fathoms; 

 bottom temperature, 2*4° C. ; grey ooze. 



Station 334. March 14, 1876. Lat. 35° 45' S., long. 18°31'W.; 1915 fathoms; 

 bottom temperature, 1'5° C; globigerina ooze. 



^Cystechinus vesica (Pis. XXXV., XXXV.« figs. 5-8; PI. XXXIX. fig. 27; PL XLL 

 figs. 36, 37; PI. XLIIL figs. 9-12; PI. XLV. figs. 32-35). 



Cystechinus vesica, A. Agassiz, 1879, Proc. Ain. Acad., vol. xiv. p. 209. 



This species is distinguished by the large size of the ambulacral plates of the abactinal 

 side, the uniform structure of the ambulacra on the actinal side, and the regular arrange- 

 ment of the coronal plates on the abactinal side of the test. 



The anal system is made up of numerous irregularly-shaped plates (PI. XXXV. figs. 

 9, 10). The ampulla3 (PL XXXV. fig. 13) of the tufted actinal tentacles (PL XXXV. 

 fig. 20) round the actinostome are elongate. The majority of the primary spines are 

 solid, straight, longitudinally striated (PL XXXV. fig. 14), with a broad base or else 

 smooth and cylindiical. A few large club-shaped primary spines (PL XXXV. fig. 15) 

 coarsely striated at the extremity and slightly curved were also left. The smaller miliary 

 spines are also club-shaped, slightly curved, and with serrated edges at the extremity. 

 The apical system is disconnected, supernumerary interambulacral plates separating the 

 bivium from the trivium (PL XXXV.^ fig. 5) ; there are three genitals (PL XXXV. figs. 

 6, 7), forming small grape-like clusters attached closely to the apical system ; the mad- 

 reporic body is large. Pedicellarise of three kinds, one of which is similar to the 

 Clypeastroid pedicellarise of Pourtalesia figured in the Revision of the Echini, the second 

 kind are large, trifid, triangular and short-stemmed (PL XXXV. fig. 16), the third 

 kind are short-stemmed and club-shaped (PL XXXV. figs. 17, 18). 



The actinal system is covered by a small number of large triangular plates, extending 

 from the actinal edge of the test to the actinostome (PL XXXV. fig. 12); on the abactinal 

 surface of the test the tubercles are not as closely placed as upon the actinal side ; we find 

 only a small number of primary sj)ines irregularly scattered over the test (PL XXXV. figs. 

 1,2); the miiiaries and secondaries are few in number and irregularly scattered over the 

 coronal plates. Actinostome not labiate, circular (PL XXXV. figs. 8, 11, 12), wdth a 

 slightly, raised edge internally corresponding to the auricles in the regular Echinids and 

 Clypeastroids. In this species the relationship of this genus with the Desmosticha and 

 Clypeastroids is quite striking. The large ambulacral plates, the barely disconnected 



