FROM THE CORAL RAG. 319 



Test nearly oval, broader before than behind, upper surface convex, more or less 

 depressed; sides rather tumid, front border slightly grooved, posterior border feebly 

 truncated, ambulacral areas of unequal vi^idth, posterior pair the widest, anterior ambulacral 

 summit nearly central, posterior situated nearly mid-way between the apical disc and the 

 vent ; base flat anteriorly and laterally, convex behind ; mouth-opening small, circular, 

 situated at the anterior fourth of the base, tubercles small, placed at some distance apart, 

 three or four concentric rows on each large plate. Anal opening oval, situated in the 

 upper part of the posterior border, about midway between the ambulacral arch and the 

 basal angle, 



Bmensions. — a. Height, one inch and one twentieth ; transverse diameter, one inch 

 and nine tenths ; antero-posterior diameter, two inches and one tenth, b. Height, one 

 inch and one twentieth ; transverse diameter, one inch and three quarters ; antero-posterior 

 diameter, one inch and nine tenths, c. Height, eight tenths of an inch ; transverse 

 diameter, one inch and four tenths ; antero-posterior diameter, one inch and a half. 



Description. — It is rather remarkable that so much confusion should have arisen 

 regarding the only two CoUi/rites figured by Leske, and that a complete transposition of 

 his specific names should have been the result of this mistake. The urchin now under 

 consideration is, doubtless, the Spatangites bicordatus, Leske, which was erroneously 

 identified by Parkinson with the Spaiangites ovalis, Leske, this formed the starting point 

 of the error, which has been faithfully copied by succeeding authors until corrected by 

 M. Desor, who has given, in his ' Synopsis,' the correct synonymy of this species. A single 

 character, which has been well represented by Leske, determines this point, namely the 

 position of the posterior ambulacral summit, in Spatangites bicordatus it is between the 

 apical disc and the vent, whilst in Spatangitis ovalis it is immediately above the anal 



The general form of Collyrites bicordata is nearly oval, the anterior half is more 

 enlarged than the posterior half (fig. 2 a, b), the upper surface is uniformly convex 

 (fig. 2 c), and the base is flat (fig. 2 b, c), the anterior border forms the segment of a much 

 larger circle than the posterior border, and it has a median depression which extends to 

 the mouth in which the single ambulacrum is lodged (fig. 2 a), the sides are slightly tumid 

 and the posterior border feebly truncated, the greatest width of the test is behind the 

 antero-lateral ambulacra, from which point it gradually tapers backwards (fig. 2 a). 



The ambulacral regions are of unequal width, the three anterior areas are about the 

 same diameter on the sides, but the posterior pair are one fourth larger, the single area is 

 the shortest; in specimen c it measures H inch, the antero-lateral li^^ inch, and the 

 postero-lateral Ixo iiich in length from the peristome to the apical disc, the ambulacral 

 plates are narrow, and on the sides of the anterior pair there are from six to seven plates 

 opposite each large interambulacral plate (fig. 2 e) ; at the base they become much wider 



42 



