42 THE FOSSIL ECHINOIDEA 



Illustrations of the Species in Plate X. 



Fig. 5. Dictyopleurus d'Archiaci: natural size. 



6. A portion : magnified. 



7. A weathered specimen : natural size. 



8. A portion : magnified. 



Genus AEACHNIOPLEUEUS, gen. nov. 



The test is circular in marginal outline, depressed, tumid at the ambitus, and rising 

 but slightly above it. Peristome deeply sunken and small, and cuts small. Apical 

 system large, deficient in details. Ambulacra narrow ; pores uniserial, in slight curves, 

 penetrating ridges which are continuous with tubercles on the scrobicular circles. 

 Tubercles small, in vertical series, perforated and crenulate, with very large scrobicules 

 raised above the test ; small secondaries in a single row on the raised scrobicular circle, 

 and radiating ridges connecting them with the boss. Small nodulose ridges or ribs 

 uniting the secondaries of each vertical series to those on the scrobicules of the adjoining 

 vertical series and also to the poriferous zone. Plates and sutures visible between the 

 ridges. 



1. Abachniopleueus eeticulatus, sp. nov. Plate IX, Figs. 6-8. 



The test is small, depressed, faintly conical abactinally, depressed actinally, with a 

 small deeply-sunken peristome with small cuts. 



The ambulacra have two rows of small perforate and crenulate tubercles situated close 

 to the poriferous zone. They are largest at the ambitus, and very small near the apical 

 system. Inferiorly they are closer together than abactinally, and better developed. 

 A tubercle at the ambitus has a disk-shaped scrobicule elevated above the mean height 

 of the test ; and ridges rounded and narrow, but long and shallow, in the form of costse 

 radiate from the base of the boss. One passes abactinally ; and another actinally crosses 

 the scrobicule, and reaches its margin, where it may unite with a corresponding one 

 from the tubercles in vertical series ; or it may unite with some confused costse having 

 large miliaries on them. On either side of the boss three or four costse radiate laterally, 

 one set, the longest, joining the costse of the two tubercles of the neighbouring ambu- 

 lacral plates, and the other, the shortest, pass to the poriferous zone close by. Each 

 costa is perforated by a pair of pores. A few knobs or large miliaries are on the costsB 

 here and there ; and there are spaces between the costse, large and oblique in the median 

 area, and wide between each successive boss. The plain surface of the test is seen at 

 the bottom. Near the actinostome the tubercles and costse become rather crowded, and 

 the spaces between the costse simulate (but are not) pits in the test. The pores are in 

 one series, and occupy but little space. 



The interradial areas are wide, and have two vertical series of small perforate and 

 crenulate tubercles rather widely apart. The tubercles are larger than those of the 

 ambulacra, are largest at the ambitus, and are most crowded towards the peristome, 



