BRITISH FOSSILS. 



3 



similar, but on the former the seccmdary granulations are more numerous 

 than on the latter. On the ambulacral plates the primary tubercles 

 are ranged in a single line, transverse to the form of the test, giving the 

 surface a lineated aspect. In some specimens they fall so directly 

 s under each other, as to give the appearance of vertical as well as 

 horizontal granulations. There are about eight or nine primary 

 tubercles on a central interambulacral, and three on an ambulacral 

 plate. The line of junction of the two series of interambulacrals, and 

 sometimes of the ambulacrals in each segment, is not unfrequently 

 strongly marked. In such specimens the interambulacrals occasionally 

 exhibit a tendency to vertical carination. Opposite each plate are three 

 pair of pores in the ambulacral sulci, which are very narrow, and run 

 almost straightly down the sides. The number of pores are a multiple 

 of the number of plates by three. The pores appear to form a single 

 series, but are not really so arranged, as we may convince ourselves by 

 tracing the poriferous avenues to their junction with the mouth. As they 

 approach that orifice they exhibit a tendency to form oblique rows, and 

 at length definitely fall into ranks of threes (fig. 11). Hence the 

 distinction between the genus Arhacia (in the sense used by writers on 

 fossils) and Echinus, founded on the arrangement of the pores in a single 

 series in the former, and in many series in the latter, is a mistake. This 

 is quite as plainly seen in the so-called Arhacia monilis, where the pairs 

 of pores are ranked in fours so distinctly all over the shell that it is 

 strange such an arrangement should have been overlooked. In an 

 undescribed green-sand urchin, brought from Portugal by Mr. Daniel 

 Sharpe, the general aspect of the shell, the arrangement of tubercles, 

 &c., so closely resemble the species we are describing, that at first 

 sight they seem identical, but when the poriferous avenues are examined, 

 the pairs of pores are seen to fall into very oblique and unmistakable 

 ranks of threes throughout. The mouth is circular, rather small in 

 proportion to the base, and very slightly notched opposite the ambu- 

 lacral furrows. The anal and ovarian circle is small in proportion, 

 being about two-fifths of the breadth of the mouth. The diameter of 

 the ovarian and anal circle is to the mouth as the maximum breadth of 

 the ambulacral to that of the interambulacral segments. The anus 

 was not protected by definite and symmetrical testaceous valves. The 

 ovarian plates are subtriangular, and perforated in their centres by a 

 conspicuous oviducal pore. One of them is in some specimens slightly 

 larger than the rest, and exhibits traces of a madreporiform tubercle. 

 The ocular plates are small, and angularly reniform ; the perforation 

 for the ocelli is in the central angle of their excavated and outer mar- 

 gins, thus presenting a slight abnormality of position. The ovarian 

 circle is exactly the diameter of the widest and most central portion of 



