S'2 SVEN LOVEN, ON THE ECHINOIDEA DESCRIBED BY LINNtEUS. 



The test is orbicular, rarely slightly pentangular, from 

 the ambulacra being faintly prominent. Vertically it never 

 is globose, generally hemisphEerical or slightly flattcned, the 

 heicrht varyino- in adult or nearly adult specimens of the dif- 

 ferent speeies between 0,6 3 and 0,4 5 of the diameter. The 

 young are more flattened. The basis is flattened, in some 

 speeies moderately so, with the lateral flexure rounded. In 

 the former case the interradia are dorsally more or less con- 

 vex, in the latter they are slightly sunk in the middle. At 

 the ambitus the breadth of the ambulacra is to that of the 

 interradia as between 30 and 46,6 to 100, the lower percen- 

 tage being fonnd in the speeies with a flattened underside. 



The stoma is large, proportioually wider in young speci- 

 mens, pentagonal, the ambulacra, bilobate, forming its sides^ 

 the interradia, emarginate, making its angles. There are no 

 incisions for the peristomal gills, but these are supported, on 

 each interradium, on two tongue-shaped lamellae, exserted from 

 the interiör, laid back, inside out, and cemented to the test 

 so as to conceal the marginal portions of the first plates with 

 a part of their tubercles; in the middle they join into a more 

 or less projecting nib. The niche for the singie spherid is 

 placcd close över the middle sinus of the ambulacrum. 



The Arbacite are orthoproctic and the retrograde tendency 

 of the periproct very little modifies the calycinal system. In 

 the youngest specimens all the radials are at equal distance 

 from it, and in full-grown specimens the I, V and IV alone 

 are perceptibly approached. Consequcntly the periproct is 

 irregularly oval, its longest diameter extending from a point 

 a little in front of the suture of the costals 3 and 4 to the costal 

 1, rather behind its middle line. The closing valves are nor- 

 mally and almost invariably four, two on each side of the 

 longer diameter; anomalously there may be three only or one 

 or two of them may be divided, or in part broken up into 

 small pieces, as in the anal membrane of most other Echinids. 

 The radials present the occurrence, elsewhere rare, of double or- 

 bits (s. v. v.) placed adorally and separated by a process^). 

 The whole calycinal system is not very large, generally low 

 and even, but covered with the same superficial layer that 

 forms the exteriör of the disks and other parts of the up- 

 per side. 



') Etudes, 9. 1. Echinoidées, p. 66, f. 7, and p. 67, fig. 1—5. 



