REPORT ON THE ECHINOIDEA. 149 



'"Cystechinus clypeatus (PL XXXV.'' figs. 10, 11; PI. XLIL figs. 15, 16; PI. XLV. 

 figs. 29-31). 



Cysteehinvs clypeatus, A. Agassiz, 1879, Proc. Am Acad., vol. xiv. p. 208. 



Numerous fragments of a large species of Cystechinus were collected, some of whicli 

 were fragments of tlie aliactinal surface with the anal system well preserved. This 

 showed a difi"erent arrangement of plates from those of either Cystechinus vesica or 

 Cystechinus loyviUii, both in the apical system and in those of the coronal plates of the 

 abactinal region. The terminal coronal plates are larger and form rows, the inter- 

 ambulacra and ambulacra consisted evidently of fewer plates than in the other species of 

 the genus. The abactinal system resembles closely that of Cystechinus imjvillii; the 

 genital plates are, however, proportionally larger, the left anterior and the right iDosterior 

 far exceeding the others in size, and extending entirely across the abactinal area, the 

 whole central jjart of which is formed by the junction of the genital plates. 



The madreporic body is more concentrated, than in any other species of this genus, 

 forming a sharp, well-defined triangular button on the right anterior genital plate. There 

 are four genitals all equally developed, and, in the fragments preserved, all greatly 

 expanded, forming huge clusters round the apical system. A piece of the aetinal surface 

 near the actinostome shows that in this species the aetinal surface is more closely 

 covered with primary tubercles than the other parts of the test. The spines are 

 whitish, long, slender, cylindrical, somewhat curved, the shaft slightly swollen towards the 

 anterior extremity beyond the middle. The miliaries are quite numerous, carrying short, 

 straight, sharp cylindrical spines. Near the actinostome the primary spines are short and 

 club-shaped, and the whole space between the primaries is thickly covered with secondary 

 spines and short-stemmed pedicellarise, which are large-headed, pyramidal, and rounded. 



The aetinal membrane and the immediately adjoining parts of the test are thickly 

 covered with short miliary club-shaped spines. The large elliptical anal system is placed 

 with its longest diameter vertically, and is protected by numerous irregularly-shaped 

 plates (PL XXXV.'' fig. 11) carrying miliaries; the anal opening is small and placed 

 centrally. The edge of the test adjoining the anal system is thickly covered by miliaries 

 forming a broad band, with an indistinct outer edge (almost a fasciole) surrounding it; 

 this band of miliaries gradually passes into the miliary tuberculation of the adjoining 

 part of the test. 



The test of this species is quite stout, judging from the thickness of the fragments 

 preserved, and measuring similar parts of Cystechinus ivyvillii; this species must have 

 attained at least 200 mm. in diameter. In the specimens from the greatest depths at 

 which this species has been found, the test is much thinner than in the fragments which 

 are found near the 1000 fathom line. 



As a general rule among the Pourtalesise, the test of the diff"erent species ha\'ing an 

 extended bathymetrical range varies very materially in thickness, according to the depth 



