210 ORGANIC REMAINS. 



each petal to the margin and the base. The spines are not tapering 

 and sharp, but cylindrical and blunt. They are most slender, and 

 most closely set, in the petals; and are longest in the middle or 

 deepest part of each petal. Each of the spaces between the petals 

 is divided by a Avaved or zigzag suture, jsassing from the vertex to 

 the margin, and giving off lateral sutures at each of its angles, in an 

 alternate form. The entire shell has been heart-shaped, having a 

 sinus or depression at its broadest end; and one of the petals points 

 to this sinus. It corresponds nearly with the echinite figured in 

 Plot's Oxfordshire, Tab. III. No. 1 and 2; but the latter is not 

 cordated, but oval, and has its central part not round but oblong. A 

 species of this kind, with raised points instead of pores, is named by 

 Linnaeus echinus pustulosus; but the reference is erroneously made to 

 Tab. II. Fig. 14 of Plot, a fossil similar in shape, but with avenues of 

 pores. Our specimen is wholly composed of a light grey flint, and 

 the small spines are therefore translucent, and have a whitish ap- 

 pearance. The base is very imperfect, but it appears to have been 

 marked as in Dr. Plot's specimen, Tab. III. Fig. 3. 



Of the third class of echinites, the pleuroci/sti, that have their vent 

 in the side, or on the upper surface, some species also occur in our 

 district. No. 2, which is the cor angidnum of Linnaeus, occurs in the 

 chalk. It is shaped like a heart, and is marked on the upper surface 

 with a flower of five petals, sunk or grooved ; each petal having two 

 double rows of pores connected by minute cross lines, the whole con- 

 verging at the termination of the petal. The largest petal passes into 

 a furrow that forms the depi-ession of the top of the heart, and is the 

 only petal that reaches the margin. It is partly continued on the base, 

 towards the mouth. Directly opposite this petal, and between 

 the two shortest petals, is a keel or elevated ridge, passing from the 

 vertex to the vent, which is at the point of the heart, in the margin. 

 The rest of the surface, between and beyond the petals, is adorned^ 



