218 STOMECHINUS. 



tubercles disposed irregularly on the surface on each plate, the number diminishing on the 

 upper surface ; tubercles surrounded by small, depressed, ring-like areolas, scarcely any 

 granules on the surface of the plates ; apical disc small, genital plates narrow, ocular 

 plates prominent, with transverse eye-holes ; base flat, inter-ambulacra forming convex lobes 

 around the peristome. 



Dimensions. — Height, seven tenths of an inch ; transverse diameter nine tenths of an 

 inch. 



Description. — This small conoidal Echinite in many respects resembles Stomechinus 

 gyratus, Agass., and might at first sight be supposed to be a mere variety of that form, 

 but a closer examination proves it to be distinct ; the test is circular and elevated, the sides 

 rising suddenly from the basal angle ; the ambulacral areas are flat, having two rows of 

 tubercles regularly arranged on their margins, about twenty-four in each row, with two 

 irregular internal rows, which occupy about the middle half of the area ; the tubercles are 

 all nearly of the same size ; they have narrow, sunken areolas around their base, but 

 scarcely any granules on the surface of the plates (fig. 2c?). 



The poriferous zones are wide, the trigeminal ranks form angles of 40° to 50° ; between 

 each rank there are two granules, and there are three ranks opposite two tubercular plates ; 

 as there are eighteen plates in each column, there are therefore about twenty-seven trige- 

 minal ranks in each poriferous zone. 



The inter-ambulacral areas are rather more thafl twice the width of the ambulacral, 

 each column contains about eighteen plates, and on each plate, at the widest part of the 

 area, there are from four to five tubercles, so irregularly disposed that the arrangement is 

 different on every plate (fig. 2 <:/); the tubercles are all about the same size, they are sur- 

 rounded by narrow, sunken areolas ; a few smaller tubercles are scattered among them, 

 but there are no circles of granules around their base, which gives the surface of the 

 plates a naked appearance; there is no depression in the middle of the areas, and the 

 region of the centro-suture between the five uppermost pairs of plates is naked but not 

 depressed ; the areas become convex and prominent below. 



The apical disc is small (fig. 2 a) ; the genital plates are narrow, and the anterior 

 larger than the posterior pair ; the spongy madreporiform body occupies the surface of the 

 right anterior plate (fig. 2 e) ; the ocular plates are small, but prominent, and the eye-holes 

 form transverse slits on their sides ; the vent is oblong (fig. 2 <?), and around its margin 

 there are three parts of a circle of granules ; the oviductal holes are large, and perforated 

 near the apex of the plates. 



The base is much covered with adhering matrix (fig. 2 /;), so that the form of the 

 mouth opening is nearly concealed ; the tubercles are all much larger in this region, and 

 the wide depressed poriferous zones, and convex inter-ambulacral lobes, impart a lobed 

 appearance to the circumference of the mouth. 



