4 



BRITISH FOSSILS. 



separating each poriferous groove is also tumid and studded with a 

 single row of conspicuous close-set tubercles, usually five or six in each 

 series. 



The genital disk is composed of four developed genital plates, each 

 with an oviducal hole. The posteal or fifth plate is undeveloped. The 

 left antero-lateral is much larger than the others ; in its upper portion, 

 which occupies the centre of the disk, it is minutely punctated, forming 

 the madreporiform body. The eye-plates are all five developed and 

 perforated. 



The whole of the dorsal and lateral plates are minutely granulated 

 and studded in a scattered manner with small, nearly equal spiniferous 

 tubercles, each consisting of a perforated tubercle placed on a crenu- 

 lated base, and surrounded by a smooth ring. They become much 

 more closely set on the lower parts of the side, and are much larger on 

 the ventral surface of the body, and regularly arranged on the ovate, 

 acute post-oral spinous space, but are few on the ambulacral plates 

 bounding that area. The ambulacral pores radiating from the mouth 

 are lodged in short inconspicuous rather rugose grooves. The sutures 

 of the plates below and near the anus are edged with minute granules 

 arranged in rows. 



The spines are rarely seen, and have never before been represented, 

 nor, I believe, described. They are very slender and small ; not, 

 however, all minute, for some are as much as a quarter of an inch in 

 length, and were possibly longer. They are mostly arcuated near the 

 base, ridged along their length by about six ribs, which in some speci- 

 mens appear to be rugose with prickle-like projections apparently ar- 

 ranged spirally, thickened suddenly at their bases, where they are sur- 

 rounded by a strongly crenulated ring. The habits of the animal, as 

 indicated by these spines, probably resembled those of the existing sea- 

 urchins of the genus Brissus. 



Very young specimens are mostly of a more elongated form than is 

 seen, except rarely, in the adult ; but there are also tumid young ones, 

 and it is possible that the sub-depressed and tumid forms of this species 

 {cor-anguinum and rostratus)^ both of which are common and found 

 together, may be male and female individuals respectively. 



Flint-casts of the interior of this urchin are not uncommon, and 

 when in fine condition serve beautifully to illustrate the arrangement of 

 the plates. The anterior and lateral interambulacral segments of the 

 body, although very unlike, are formed of about twelve pairs of plates 

 in each compartment. Those of the anterior segment gradually in- 

 crease in dimensions as they approach the wider and lower portion of 

 the cheeks ; those of the lateral compartments are unsymmetrically un- 

 equal, and the lower or ventral ones assume irregular polygonal shapes 

 with increased size, in order to fill up the triangular sides of the oral 



