368 CLYPEUS 



have marginal eyeholes, and the surface of all the discal elements is covered with fine 

 granulations. 



This urchin, when viewed in profile, presents a considerable undulation around the 

 border (fig. 1 c and d), occasioned by the narrowness of the ambulacra, and the extreme 

 tumidity of the marginal and basal portions of the inter-ambulacra. It is as 

 remarkable for the concavity of its base, and the cushion-like structure of its basal 

 inter-ambulacra, as for the evenness and convexity of the dorsal portions of the same seg- 

 ments : the basal portions of the antero-lateral inter-ambulacra are small, and moderately 

 convex (fig. 1 c and d), the postero-lateral pair are very prominent and tumid, and the 

 single inter-ambulacrum is angular, deflected, and truncated at its extremity. 



The small mouth-opening lies in a deep sub-central depression (fig. 1 li), and the 

 peristome is surrounded by five small oral lobes ; the basal portions of the ambulacra 

 are narrow, and the poriferous zones so small and indistinct that the pores, even with a 

 lens, are seen with difficulty ; as they approach the mouth they increase in size and 

 number, and form a series of triple oblique rows in the five petal-like expansions which 

 radiate from the mouth (fig. 1 6). 



The small tubercles are in general arranged in four horizontal rows on each plate (fig. 

 1 e) ; they are very uniform in size, and surrounded by sunken areolas, the tubercles 

 at the base and border are larger than those of the upper surface ; all the intermediate 

 surface of the plates is covered with fine homogeneous granules, microscopic in size, but 

 regular in their arrangement. 



JJJinities and differences. — This species resembles some of the smaller varieties of 

 Clypeus Plotii, Klein, but is readily distinguished from these, and from all congeners, 

 by its greater proportional breadth across the postero-lateral inter-ambulacra, the narrow- 

 ness and fissure-like character of the anal valley, the remarkable undulations of the 

 border, the tumidity of the basal inter-ambulacral cushions, and the concavity of the base. 

 It resembles Clypeus Miilleri, Wr., in the shortness of the petaloid portion of the dorsal 

 ambulacra, and the narrowness of the anal valley ; but the oblong form of Clypeus Miilleri, 

 the flatness of its base, and the depression of its dorsal surface, form diagnostic distinctions 

 between them. In the narrowness of its ambulacral petals it resembles Clypeus Mickelini, 

 Wr., but the flat dorsal and basal surfaces of that species form distinctive characters by 

 which the two urchins are readily distinguished from each other. 



Locality and StraiiyrapJiical position. — I have collected this urchin from the 

 Inferior Oolite of Dorsetshire only, in the upper ragstones of that formation, appertain- 

 ing to the zone of Ammonites Parkinsoni, as defined in the chapter on the Strati- 

 graphical distribution of Oolitic Echinodermata. I have found it at Burton-Bradstock, 

 and Walditch Hill, near Bridport, associated with Ammonites Parkinsoni, Sow., Ammonites 



