32 ROBERT TRACY JACKSON OX ECHIXT. 



quite spherical, Maccoya sphaerica (Plate 32, fig. 5), most species of Lovenechinus and Melon- 

 echinus (Plate 55); or depressed spheroidal, as in Archaeocidaris and Cidaris. 



In all the above types the ambitus coincides very nearly with the mid-zone. The hori- 

 zontal section in any zone may be nearly or quite circular, or modified by more or less pro- 

 nounced melon-like ribs or elevations of both the ambulacra and the interambulacra, as in 

 Melonechinus (Plate 60, fig. 3). The test may be depressed spheroidal with ambitus below 

 the mid-zone, as in Lepidocentrotus ivhitfieldi (Plate 19, figs. 6, 7), or recent Strongylocentrotus. 

 Rarely the ambitus is above the mid-zone, as in the high obovoid Lepidesthes corcyi (Plate 66, 

 figs. 8-10) and Melonechinus obovatus (Plate 54, fig. 2). As far as I am aware, this character 

 of ambitus above the mid-zone is known otherwise typically only in the recent depressed Echino- 

 strephus molare (Blainville), in which Mr. Agassiz (1873, p. 457, Plate 5a, fig. 11) showed that 

 the greatest diameter is near the abactinal surface. It may occur occasionally as a variation, as 

 I found in three specimens of Strongylocentrotus purpuratus out of 120, and in one specimen of 

 Strongylocentrotus drohachiensis in 33,000. It is undoubtedly an exceptional character for the 

 ambitus to be above the mid-zone in fossil or Recent Echini. The test may be depressed with 

 strongly pentagonal outline, as in Hyattechinus pentagonus (Plate 24, figs. 1-4). It may be 

 circular but greatly flattened dorso-ventrally, as in Hyattechinus rarispinus (Plate 22), which 

 apparently closely approached Echinarachnius in form. Or finally, in Palaeozoic types, the 

 test may be flat on the base with rounded dorsal side, of moderate bilaterality, almost Cly- 

 peaster-like in form, as in Hyattechinus beecheri (Plate 24, figs. 5-8). This includes all the 

 forms of tests known in the Palaeozoic species. 



The test may be radially equal with central periproct, as are most Palaeozoic Echini and 

 most post-Palaeozoic regular Echini except Echinometra and allies. Or it may be bilateral 

 with central periproct, Hyattechinus beecheri (Plate 24, figs. 5-8; Plate 25, fig. 5), a character 

 not known to me in any other Echini. Or it may be more or less regular in form but with 

 the periproct eccentric in an interambulacrum, which is probably the odd posterior interambu- 

 lacrum, Echinocystites. 



Orientation. 



The matter of orientation of a sea-urchin in relation to its true axis is important, and has 

 been briefly touched upon in the Introduction. Loven (1874) urged that the proper orienta- 

 tion of an echinoid is to take as the antero-posterior axis a line drawn through an ambulacrum 

 and opposite interambulacrum, in such a plane that the madreporite lies in the right anterior 

 interambulacrum. It seems rather arbitrary in the regular modern Echini at first sight, but 

 there are good reasons for the acceptance of this orientation here. The most obvious reason, 

 as pointed out by Loven, is that it is morphologically the same line on which bilaterality is 



