REPORT ON THE ECHINOIDEA MORTENSEN 267 



narrow belt of small tubercles outside the scrobicular ring. On the 

 adradial edge of the plates there is barely room for a single series of 

 small tubercles alternating with the scrobicular ones. The groove at 

 the median end of the horizontal sutures is fairly conspicuous, merg- 

 ing into the sunken vertical median part; also at the adradial end of 

 the sutures there is a rather distinct indication of a groove. The 

 whole of the horizontal suture between the uppermost interambulacral 

 plates may be more or less distinctly sunken. The median area of 

 the interambulacra is scarcely two-thirds the width of an areole. 



The apical system (fig. 10) is usually a little more than half of the 

 horizontal diameter, rather thin, and not raised above the level of 

 the test; it sometimes forms a slight rounded elevation, which may 

 almost be termed subcorneal; more often the periproct alone is a 

 little elevated, the genital and ocular plates remaining flat. The 

 oculars are of a characteristic triangular form, with an acute apex 

 inward, which often reaches the periproct, the oculars thus being gen- 

 erally, but not always, narrowly insert. The female genital pores 

 are large and near the edge of the plates, the male pores being much 

 smaller and about in the middle of the plates. The madreporite is 

 not enlarged. The whole apical system is usually pentagonal in out- 

 line. The periproct is rather small. All the plates of the apical 

 system are rather sparsely covered with small tubercles of uniform 

 size; even those on the inner edge of the periproctal plates are not 

 larger, corresponding to the fact that the secondary spines surrounding 

 the anal opening are not larger than the other spines on the apical 

 system, contrary to what is otherwise very generally the case in cida- 

 rids. A rather broad edge is left bare on the genital plates. 



The genital pores are about to appear in a specimen 9 mm. in hori- 

 zontal diameter. 



The peristome is distinctly smaller than the apical system, some- 

 what irregularly pentagonal in outline. There are only 5 or 6 ambu- 

 lacral plates in a series. The ambulacra do not join at the mouth 

 edge, thus leaving a free though narrow passage for the interradial 

 plates, which are more or less irregular, 2 to 4 in number. The peri- 

 stomial plates in general are rather delicate. 



The primary spines are long and slender, about three times the 

 horizontal diameter, the ambital ones usually rather distinctly curved 

 downward. They taper to a rather fine point. The basal disk is 

 only indistinctly developed; usually there are only some larger spinules 

 at the base which may be more or less flattened and broadened, so as 

 to be united to some slight degree across the adapical side of the spine. 

 But often these spinules are lacking, and then there is no trace of a 

 basal disk. The shaft of the spine is otherwise set with rather sharp, 

 outstanding spinules which are not curved outward, which show no 

 regular serial arrangement, and which do not increase in size toward 



