PROM THE WHITE CHALK. 273 



Leske and others. It presents a great many forms which have been considered distinct 

 species by some, but only as varieties by others. The specimen I have selected for 

 figuring and description is a capital type of the species in the highest state of preserva- 

 tion, and the anatomy of the test has been carefully displayed in a series of figures in 

 my late lamented friend Mr. C. R. Bone's best manner. 



The test is cordiform, and its width and length are nearly equal ; it is enlarged before 

 and contracted behind, and its greatest transverse diameter is at the junction of the 

 anterior with the middle third ; the upper surface is inflated ; from the disc to the anterior 

 border it is obliquely depressed, fig. 1 c, and from the disc to the posterior border it 

 describes a curved line which forms the summit of a carinal ridge extending to the vent 

 and forming a rostrated process at the upper border of the periprocte ; the sides are much 

 inflated, fig. 1 a, and the posterior border is fiat, obliquely truncated downwards and 

 inwards, and with the rostrated carina forms an excavation which imparts a specific 

 character to the species, which is well delineated in fig. 1 c. 



The pairs of petaloid ambulacra are of unequal length, the anterior pair being the 

 largest ; they are not much depressed and very slightly flexed ; the poriferous zones are 

 narrow, not rounded at their extremity, and present certain characters which prevail 

 throughout the various forms this species exhibits. 



The ambulacral plates are invariably tumid, rugose, and separated by a strongly 

 marked suture, fig. 1 h, magnified four diameters ; whilst the general suture running down 

 the centre of the ambulacral area is deeply depressed and grooved like fig. 1 a ; the ridge 

 separating each poriferous groove is also tumid and studded with a single row of five or 

 six small close-set tubercles, fig. 1 h. 



The single anterior ambulacrum is feebly petaloid in its upper part ; its pores are set 

 much closer together than those in the pairs, and in many specimens the grooves 

 connecting the pores are obsolete ; the pores of the inner row in the lateral ambulacra is 

 rounder than those of the outer row, which are slightly elongated, fig. 1 h ; the non- 

 petaloid portion of this area is more fully developed than in the corresponding portion 

 of the others ; fig. 1 i shows the anterior ambulacrum. It is lanceolate, the plates are 

 wide and irregular, each having a pair of small holes with tubercles placed between them. 

 The inter-ambulacral areas are formed of large plates eight to ten in each column 

 according to the age of specimen. They attain a greater development at the base, the 

 postero-lateral pair forming the largest portion of the region, fig. 1 b ; the antero-lateral 

 pair forming the anterior border, fig. 1 a, e ; the carinal ridge, the anal area, the central 

 shield of the base, with its circular subanal fasciole, and highly ornamented tubercular 

 surface, are all formed by the single posterior area, fig. 1 b, c, d. 



The surface of the inter-ambulacral plates is covered with highly developed tubercles 

 on the sides and upper surface ; each plate has four or five horizontal rows of them more 

 or less regularly arranged as shown in fig. 1 /, where two of these plates are drawn 

 magnified four diameters. 



35 



