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



SjxUagocystis the actinal groove is limited to the actinal surface (PI. XXVL* fig. 2), and 

 the extension of the actinal groove from the actinal side over the ambitus to the aljactinal 

 side forms but a slight indentation in the anterior part of the test (PI. XXVI. fig. 3). 



There is a small anal snout (PI. XXVI. fig. 1 ; Ph XXVI." figs. 11, 13, 15, 16), with 

 a sharp beak extending below the anal system ; there is no trace of a well-defined fasciole, 

 but there are miliary tubercles closely packed at the tip of the snout on the lower side 

 (PI. XXVI.^ fig. 12), these correspond undoubtedly to the distinct sub-anal fasciole 

 encircling the anal snout of Pourtalesia proper. This fasciole, while very distinct in 

 some species of Pourtalesia, is somewhat iU-defined in others, and thus readily passes 

 into such an indistinct accumulation of miliaries as we find in Spatagocystis, where 

 it forms the first trace of the sub-anal fasciole of the group. The anal pouch 

 (PI. XXVI." fig. 6) is much shallower than in Pourtalesia proper. 



The test seen facing the posterior extremity (PI. XXVI. fig. 5) shows how prominent 

 the keel formed by the prolongation of the actinal plastron has become. The test is 

 regularly arched, and nearly as gil^bous on the actinal side (with the exception of the 

 somewhat flattened anterior extremity) as on the abactinal side. 



Seen facing the anterior extremity the sunken actinal groove is seen to extend but 

 little into the anterior part of the test above the ambitus (PI. XXVI. fig. 4). 



The extremity of the anal snout barely extends in the largest specimens collected 

 (PI. XXVI. fig. 1) as far out as the line of the abactinal hood over the anal system 

 (PI. XXVI.^ figs. 15, 16). In young specimens which are more elongate (PI. XXVI. 

 figs. 8, 9) and more pointed posteriorly, the actinal keel is exceedingly prominent when 

 seen in profile (PI. XXVI. fig. 7), projecting beyond the line of the abactinal hood of the 

 anal system. In these younger specimens the beak is also more limited to the posterior 

 area, and the test is more flattened on the actinal side, and less gibbous as is v?ell showni 

 in the figures seen from the anal or actinal extremity (PI. XXVI. figs. 10, 11). 



This species is remarkaljle for the great length of the narrow j)lates composing the 

 posterior lateral interambulacra (PI. XXVI. * figs. 15, 16) and the anterior zone of the 

 posterior lateral ambulacra ; the width of the other areas forming the sides of the test 

 is more uniform (PI. XXVI. " fig. 9), the median odd interambulacral zone which forms 

 the crest of the rounded part of the abactinal region of the test is very narrow (PI. XXVI." 

 fig. 17), and is composed of plates of nearly uniform size extending to the abactinal region 

 of the anal hood (PI. XXVI.-'' figs. 10, 17). This is flanked by one row of rather 

 smaller ambulacral plates forming the posterior zone of the lateral posterior ambulacra ; 

 in fig. 10 the outer plates should be perforated by the pores, and not the median rows 

 of plates, which are the plates composing the odd interambulacral zone (PI. XXVI.'' 

 fig. 17). The anterior zone of the lateral j)osterior ambulacra is composed of wider plates 

 (PL XXVI." fig. 15). The apical system corresponds to the apex placed anteriorly ; there 

 are three genital pores, the macLreporic body is distinct ; there are four genital plates 



