ECHINOCONUS. 213 



■('iig. 2d); the surface of the plates is likewise covered with numerous microscopic 

 granules set in horizontal rows. There are four ambulacral plates opposite each inter- 

 ambulacral, and a pair of oblique pores correspond to each ambulacral plate. 



The inter-ambulacral areas have two rows of primary tubercles, nearer the zonal than 

 the sutural side of the plates ; these rows extend from the disc to the peristome (fig. 2 a, b, 

 • and fig. 3 a) ; each plate besides, near the ambitus, supports four smaller tubercles 

 'placed less regular on the surface than those forming the vertical series. The surface of 

 the plates is likewise covered with a most abundant development of microscopic granules 

 arranged in horizontal rows in the direction of the long diameter of the plate. Fig. 2 d 

 shows this condition of the surface in the plates of both areas magnified six times ; all the 

 tubercles are perforated and crenulated. Those on the base are large as shown in fig. 3 a, 

 where the test is magnified twice ; and the details of a portion of the same are very well 

 exhibited in fig. 3 h, where a portion of both areas is accurately drawn eight times the 

 natural size. These admirable drawings by my friend Mr. C. R. Bone render any 

 detailed description unnecessary, as they are correct representations of the minute anatomy 

 •of the test carefully drawn under a lens. 



The apical disc is level with the general surface, and all the five ovarial plates are 

 perforated (fig. 2 c), as in D. minima. 



The base is slightly convex ; the small mouth-opeuing is quite superficial, and its 

 .peristome is marked with microscopic incisions ; the vent is oblong, and occupies the 

 middle of the space between the peristome and border, which is rounded and inflated, 

 but never angular as in B. snbuculus. 



Affinities and Differences.— ^\\Qn compared with D. suLucuIas it is distinguished by 

 •the proportions and dimensions of mouth and vent as compared with the whole ventral area, 

 which are much larger in D. subiicidus than in D. Dixoni. M. Cotteau is inclined to 

 unite D. Dixoni to D. minima, as the apical disc in both has each of the five genital plates 

 ^perforated, whereas, in other congeneric forms, only fom- of the ovarials are so. 



Locality and StratiffrapMcal Position.— The type-specimen was collected from the 

 White Chalk of Sussex, and the original of our figures belongs to the British Museum. 



Gemcs — Echinoconus, Breynim, 1732. 

 CoNULUS, ffZwi, 1734. EcHiNiTES, Xe«^e, 1778. Gtale rites, Zamarc/:, ISO 1 . 



Shell round, oval, or pentagonal ; enlarged a little before and slightly contracted 

 behind ; upper surface more or less elevated, and having a rounded or conoidal form ; 

 :summit central ; under surface flat, border rounded or angular, sometimes concave in the 

 middle, rarely undulated by the depression of the ambulacra. 



Mouth-opening small, circular, decagonal, in the centre of the base; peristome 



