268 HOLECTYPUS. 



and has the single inter-ambulacrum produced ; anal opening at the base ; tubercles on 

 the upper surface very small and indistinct ; those at the base and margin moderately 

 large ; mouth opening concealed. 



Dimensions. — Height, seven tenths of an inch ; transverse diameter, one inch and a 

 quarter ; antero-posterior diameter, one inch and four tenths. 



Description. — The few specimens of this urchin I have examined were not in good 

 preservation, the surface of the test having been more or less rubbed in them all, so much 

 so, that a detailed description, with the materials at my disposal, is at present impossible. 



The test is oblong, with tumid sides ; it is rounded before, contracted and elongated 

 behind, so that the anterior half is an eighth of an inch shorter than the posterior half, 

 when measured from the disc to the border (fig. 3 a). 



The ambulacral areas are one third the width of the inter-ambulacral ; they have six 

 rows of small tubercles, which form V-shaped lines throughout the area ; the poriferous 

 zones are narrow ; and there are five pairs of pores opposite each inter-ambulacral plate 

 (fig. 3 d). 



The inter-ambulacral spaces are of equal width, and three times as broad as the 

 ambulacral (fig. 3 a) ; from the disc to the border there are twenty plates in each column, 

 which are all slightly bent in the middle ; on some of the plates there is only one row of 

 tubercles, but in the others there are two rows, arranged as in fig. 3 d, which are small 

 and indistinct from friction ; the basal tubercles of both areas are larger, and they are 

 arranged in single concentric lines. 



The base is much concealed in all the specimens I have seen ; the mouth is entirely, 

 the vent partially so, in those before me ; the anal opening appears to be a large aperture, 

 situated between the mouth and the border, but encroaching a little upon the latter ; and 

 has the apex directed towards the mouth. 



The apical disc is small, vertical, and excentral, being nearer the anterior than the 

 posterior border ; the ovarial plates are small, with the exception of the right antero- 

 lateral, which is nearly twice as large as the others, and supports an oblong, madreporiform 

 body ; the ocular plates are very small, and perforated near their border. 



Affinities and differences. — This species very much resembles H. depressus, Leske ; but 

 it has a more oblong form, more tumid sides, and the anal opening is nearer the 

 border than in that species ; by the smallness of the tubercles it is distinguished from 

 H. corallinus, d'Orbigny, and by the arrangement of the tubercles on the plates from 

 H. depressus ; the basal position of the anus separates it from H. hemisphcericus, Agassiz. 

 The imperfect condition of all the specimens I have seen renders a more critical comparison 

 at present impossible. 



Locality and Stratigraphical position. — The four or five specimens I have examined 

 were all collected from the Coralline Oolite of Malton, Yorkshire. 



