210 



PAXAMIC DEEP SEA ECHINI. 



The anal system is cut out of only three interambulacrpl plates 

 (PI. 105, fig. .?). 



The apical system of F. tenuis, Fig. .305, is very similar to that of 

 Hemiaster hufo, Fig. 304 ; it has four genital plates in contact with four 



Fig. 304. Hemiaster bufo. 



Afxeb LoviN. 



Fig. 305. Periaster tenuis. 



genital openings, with a madreporite limited to the right anterior genital 

 (PI. 105, fig. 2)y though it extends so as to come in contact with the left 

 posterior genital plate. 



The actinostome is paved by numerous longitudinal rows of small plates 

 radiating irregularly towards the labium (PI. 105, fig. 1). The anal system 

 is bordered on the upper edge by four large polygonal plates ; those 

 covering the rest of the anus are smaller. 



There must have been some mistake in the identification of the Schiz- 

 asterid collected by the "Challenger" (PI. XXXV \ figs. 1-4) as Periaster 

 limicola. A comparison of the figures on PI. XXXV ^, of the '' Challenger" 

 Ecbinoidea with those collected by the " Blake " off the mouth of the 

 Mississippi ^ shows great differences in outline both in profile and from 

 above. In profile the species from the Gulf of Mexico (PI. Ill, fig. l)Ms 

 seen to be quite rectangular in outline, and posteriorly vertically truncated, 

 with a rounded anterior extremity ; while the " Challenger " specimens 

 (PI. XXXV **, fig. 3) are more rounded anteriorly, and the posterior ex- 

 tremity slants from the anal system towards the actinal side. 



Seen from above the original P. limicola is quite circular, slightly in- 

 dented at the ambulncral areas (PI. Ill, figs. 2, 3)' while the "Challenger" 

 specimens are ovoid (PI. XXXV*, figs. 1, 2). But by far the most impor- 

 tant differences are those shown by the short labium, the wide sternum, 



1 BuU. M. C. Z. 1878, No. 9, Vol. V, p. 193, PI. III. 



