GEKEKA AJ^D GROUPS OF THE ECHINOIDEA. 63 



Ambulacra narrow, with very small perforate and plain primary 

 tubercles in two rows ; pairs of pores in simple vertical series, 

 rarely, but sometimes, crowded at the peristome ; plates low, 

 simple primaries near the apical system, and compound elsewhere ; 

 the adoral constituent a small primary with its aboral suture 

 convex ; the median plate a large primary, and the aboral plate 

 a low primary with the adoral suture convex (diadematoid). 



Interradia with four vertical rows of small perforate, plain, 

 primary tubercles, without scrobicules, rather larger than those 

 of the ambulacra ; the outer vertical rows reach up furthest ab- 

 actinally, and hence a granular median area. 



Peristome large, branchial grooves moderate. Spines longer 

 than the diameter of the test, very delicate and slender, sharp, 

 striated longitudinally, solid. 



Fossil. Infra- Lias and Lias : England and Europe. 



Etallon remarked long since upon the slight value of the 

 genus ILemipecUna and noticed that crenulation of the tubercles 

 is sometimes visible on some portions of a test. Hence the plain 

 condition is in this, as in other groups of forms, not of generic 

 value. It is not possible to separate Semipedina from Biadema 

 generically, but the species formerly associated with it form a 

 somewhat natural series, and may enter a subgenus. 



Subgenus Hemipedina, Wright, 1855 (genus), Pal. Soc. Monogr.., 

 EcJi. Foss. Oolit. Form. p. 143. A. Agassiz, 1872-74, Re- 

 vision, p. 291. Du7ican, 1885, Quart. Journ. Geol. Soc. 

 vol. xli. p. 42. Boderlein, 1886, Wiegm. ArcTiiv, Heft i. p. 96. 



Test moderate in size or small, circular or slightly pentagonal 

 at the tumid ambitus, flat actinally, .depressed, tumid, or sub- 

 conical dorsally. Coronal plates numerous. 



Apical disk large, with large united basal plates. 



Ambulacra narrow, straight ; pairs of pores in straight series or 

 in arcs of three ; simple primary plates near the apex, compound 

 plates elsewhere as in Diadema ; tubercles in two vertical rows 

 perforate and not crenulate ; secondary tubercles exist and 

 granules. 



Interradia large, and with from two to six vertical rows of 

 small primary tubercles, but larger than those of the ambulacra, 

 only the outer rows reach the apex, perforate, and some may be 

 crenulated. Median area often bare or granular near the apex ; 

 secondary tubercles form rows near the ambulacra. 



