298 CARDIASTER 



Stage on the Continent. The test is largely cordiform, having the length and breadth of 

 nearly equal diameters ; the anterior border is convex, with a deep anteal sulcus, having 

 sharply defined carinal borders extending upwards towards the apical disc ; the summit 

 of the ridges forming the sides of this sulcus attain the highest point in the test. The 

 posterior half of the shell tapers considerably towards the posterior border, which is 

 narrow, truncated, and depressed. The sides and border are slightly angular from the 

 way the broad plates forming the interambulacra areas are bent, and this angularity 

 imparts a very marked character to the tests of this Urchin. Pigs. 1 a and c show these 

 angles fairly well. 



The ambulacral areas are of unequal width ; the anterior pair are the longest and 

 largest, and extend across the test ; their poriferous zones are unequal, as the inner series 

 have the pores more nearly approximated than those of the outer series ; the apertures 

 have the form of narrow slits, and the pairs of pores are placed close together in the 

 upper part of the zones, but become wider asunder as the discal distance increases, so 

 that at the middle of the side they are much wider apart, and on the lower part of the 

 ambitus quite remote from each other (figs. 1 a, c). 



The posterior pair of ambulacra are much shorter and directed backwards ; the pores 

 in their zones are likewise in the form of slits, and become in like manner wider asunder 

 as they descend the area. The ambulacra and poriferous zones are in no way depressed, 

 and occupy the surface of the test (figs. 1 c, d). 



The inter-ambulacral areas are very wide, and built up of long plates, which are bent 

 transversely near their middle ; this bending of the plates produces the angles in the test 

 already referred to; between the antero- and postero-lateral ambulacra the angles are 

 marked out by a series of knotty lines along the sides of the test, and a similar ridge 

 runs between the antero-lateral ambulacra and the prominent sides of the anteal sulcus, 

 so that the angularity of the test is an interesting character of this species. 



The upper surface is covered with very minute tubercles, which become a little 

 larger on the anterior border and are still larger on the sides of the anteal sulcus. 



The apical disc is small, and well soldered into the plates of the test ; there are four 

 perforated ovarials, and five ocular plates. 



The under surface is convex and angular ; the plastron forms a prominent ridge, 

 which divides the posterior half along the mesial line, and it becomes concave at the 

 anterior third where the mouth is situated (fig. 1 b) ; the basal plates are covered with 

 larger tubercles, which have a well-defined arrangement on the plastron and on the 

 lateral parts of the base (fig. 1 b). 



The mouth-opening is near the anterior border ; it is a wide opening, transversely 

 oblong, with a well-defined peristome and rudimentary labial process. 



The vent is oval, and situated in the upper part of the posterior border, which is 

 depressed (fig. 1 d) ; two ridges extend vertically along the sides of the border, and 

 increase the depth of the concavity in which the periprocte is placed, producing a beak- 



