380 THE FOSSIL ECHINOIDEA 



The widest part of the interporiferous area lies within the apical third of the petal and 

 measures 3-5 millim., and the area contracts very slightly towards the apex. 



The interradial areas are very narrow and band-like near the apex, and are 

 extremely narrow on the actinal surface. The ornamentation consists of very small 

 scrobiculated tubercles widely spaced, the intervening surface covered with very small 

 uniform miliary granules comparatively widely spaced. The scrobicules of the inter- 

 poriferous areas are more closely placed than in the interradial areas; and the 

 primary tubercles on the actinal surface are slightly larger than on the abactinal 

 surface, but decrease in size as they approach the median radial furrow. 



The peristome is very small and subcircular, with a faint decagonal tendency, and 

 the surrounding test is slightly but abruptly depressed towards its margin, forming a 

 steeply inclined bevel. Along the median radial lines is a well-defined but rather 

 shallow furrow, which extends from the peristome nearly to the margin, dying out 

 gradually. At the extremity of each groove in the inclined peristomial margin is a 

 pair of well-developed buccal pores ; and behind these, along the median radial line of 

 the furrow, there is frequently a uniserial line of granules extending for a considerable 

 distance from the peristome ; but they are not always present. 



The periproct is small and circular, very nearly as large as the peristome, and the 

 space between the outer edge of the aperture and the posterior margin of the test is 

 exactly one third of the distance between the margin and the peristome. 



In broken tests the internal columnar walls running parallel to the margin, which 

 are characteristic of the genus Laganum, are well seen. 



Variations and young Stages. The general characters of this species are on the 

 whole remarkably constant, though naturally a certain amount of variation may be 

 observed in a lai-ge series. This chiefly afi'ects the marginal contour, and the amount 

 and character of the marginal tumidity, and is probably in great part associated with 

 the varying stages of growth — smaller examples being less strikingly pentagonal, and 

 with a more or less definite tendency to a suboval contour ; in these, again, the tumid 

 border, though still definite, is less developed, and, instead of showing the bevel to the 

 margin so characteristic of large specimens, is more tumidly rounded. In the smallest 

 examples the convexity and height of the central region of the abactinal surface is 

 proportionally slightly greater than in the adult forms, and sometimes also the con- 

 cavity of the actinal surface is more noticeable. On the whole, however, the characters 

 of the young form are so well defined, that there is no difficulty in recognizing the 

 species in small examples 15 millim. in length. 



EemarJcs. The form which appears to be the most nearly related to this species is 

 the living L. depressum, from which, however, it is readily distinguished by its greater 

 thickness, by the remarkably tumid border, by the greater prominence of the margin 

 anteriorly, and by the well-rounded posterior radial angles. 



Dimensions. Length of the test 47 millim., breadth 45 millim., height 10 millim. 

 IjQcality. Kharak Island. Survey-number G ^^. 



