FROM THE LOWER GREENSAND. 155 



eighteen in each. later-ambulacra wide, two rows of tubercles, five to six in each, largest 

 at the ambitus. Apical disc large, circular, with undulating borders. Moutli-opening 

 circular, peristome decagonal, vent transversely oval. 



Dimensions. — Height four tenths of an inch ; transverse diameter eight tenths of 

 an inch. 



Description. — This Salenia, belonging to the British Museum, is a very rare 

 form, and was referred by my late friend Dr. Woodward to P. Lardyi. The test is 

 circular with inflated sides ; the upper surface is convex and elevated, and the base is flat. 

 The poriferous zones are straight, formed of oval pores arranged in obUque pairs ; 

 separated a small granule, and unigeminal, throughout, except at the base, where they are 

 a little crowded. 



The ambulacral areas are straight, with two rows- of marginal homogenous, mani- 

 millated, granules, eighteen in a row (fig. 4 c, d) ; between the rows there are a double 

 series of microscope granulets filling in the spaces. 



The inter-ambulacral areas are large, with two rows of tubercles, five to six in 

 each (fig. 4 d), largest at the ambitus, diminishing above towards the disc, and below 

 towards the peristome ; a double row of large mammillated granules extends down the 

 middle of the area, and forms circlets around the tubercles ; a large mammillated 

 tubercle occupies the outer angle of each plate near its union with the poriferous zones 

 (fig. 4 c, d). Around the base of these granules a number of small granulets are freely 

 distributed ; the surface of the test has therefore a highly ornamented appearance. 



The apical disc is large and undulated at the border ; in our specimen (fig. 4) the 

 sutures are much filled in, and the typical character of the species is thereby obscured. 

 In the beautiful specimens from the Aptien of Auxerre the sutures are sharply engraved 

 with isolated points and lines of incision, so as to impart a parsley -leaved character to all 

 the elements of the disc. The vent is moderately wide and transversely oval, and the 

 rim of the periprocte is a little elevated. 



The mouth-opening lies in a slight depression, it is about half as wide as the diameter 

 of the test. The peristome is decagonal, and marked by feeble incisions ; of the ten 

 lobes those over the ambulacra are the largest. 



Jffinilies and Differences. — In its form and general characters P. Lardyi approaches 

 P.stelhdafus; it is, however, a larger Urchin, with its upper surface more convex and inflated, 

 sometimes it even assumes a sub-conical shape ; the marginal rows of mammillated 

 granules on the ambulacra are more numerous, and separated from each other by a double 

 series of microscopic granulets ; the apical disc is likewise thicker and more prominent. 



Locality and Stratiyraphical Position. — Collected from the Sponge-gravel of the 

 Lower Greensand, near Farringdon, where it was associated with P. Wrightii. It is a 

 very rare form in the English Neocomian strata. The fossil figured belongs to the 

 British Museum, and is the only English specimen I know. 



