186 Dr. T. Wright on Fossil Echinoderms 



Small specimeu. Antero-posterior diameter ly^^ inch, trans- 

 vei*se diameter l^J inch; height over the middle of the single 

 ambulacrum y%ths of an inch, at the highest point of the dorsal 

 region ly^^ inch. 



Description. — This beautiful Urchin is one of the most typical 

 forms of the group to which it naturally belongs. The test is 

 oblong and inclined, from the height of the anterior third being 

 less by y^^ths of an inch than the posterior third ; the ambu- 

 lacral areas are all well developed, and arranged in the form of a 

 St. Andrews cross ; as the apical disc is situated near the centre 

 of the body, the regularity of their arrangement forms a con- 

 spicuous character of this species. The anterior ambulacrum (1 a) 

 is concave, and makes an inconsiderable anteal sulcus ; there is a 

 single row of pores, flanked by a row of tubercles on each side, 

 with a space between filled by a microscopic granulation ; it is 

 abruptly bounded below by the fasciole, and terminates at the 

 disc in a blunt lancet-shaped apex. The antero-lateral ambulacra 

 in the large specimen are y^ths of an inch longer than the pos- 

 terior pair, and form an angle of 37° with the transverse line 

 through the disc ; they are round at the base and blunted at the 

 apex^ and the anterior side is more rounded off than the pos- 

 terior for the reception of the apex of the ambulacrum ; in the 

 anterior zone there are twenty, and in the posterior zone twenty- 

 four pairs of holes. The postero-lateral pair describe an angle of 

 55° ; both pairs form concave valleys ; the pores in the zones are 

 of the same size, and are pierced so wide apart (1 c) that the pores 

 of each pair are nearly as distant from each other as the width of 

 the space which separates the two avenues ; in the anterior am- 

 bulacral avenues there are twenty in the anterior and twenty-four 

 in the posterior zone; in the posterior pair the numbers are 

 twenty-two before and eighteen behind. The peripetal fasciole (1 e) 

 has an unequal width in different parts of its track ; it is narrow 

 where it passes over elevations of the test, or is bent into angles, 

 and becomes wider in other parts of its course. The apical disc 

 (1 «) is small, heart-shaped and central ; the two anterior genital 

 holes are smaller and placed closer together than the posterior 

 pair ; the five eye-holes as usual are situated at the summit of 

 the ambulacral apices : the madreporiform tubercle occupies the 

 surface of the posterior triangular genital plate. The anus is a 

 large oblong opening, situated in the upper half of the posterior 

 border, at the distance of /^jths of an inch from the fasciole in 

 the small individual. The base in both specimens is concealed; 

 the tubercles (Id) are small, crenulated and perforated, and nearly 

 of the same size ; a few larger ones occupy the sides of the an- 

 terior ambulacral sulcus ; the sides of the ambulacral areas and 

 as much of their basal portions as is exposed are destitute of 

 tubercles. 



