168 



Mr. H. Ohshima. Reversal of 



Plate 11. 



riG.'^4 and 5. — The two hammer-stones found in association with the Selsey rostrate. 



Drawn of the actual size of the specimens. That shown in the upper figure 

 (fig. 5) is less than half the bulk of the other. It is of pyramidal form, and is 

 drawn so as to show the apex of the pyramid as the centre of the picture, with 

 three large concave flake scars and a fourth convex area, radiating from it. 

 The apex shows numerous small fractures due to its use as the striking 

 surface. The larger hammer -stone drawn in the lower figure (fig. 6) has also 

 been shaped by large flakes to a pyramidal form. In this drawing the apex 

 forms the top of the figure, the stone resting on the opposite surface or base. 



Reversal of Asymmetry in the Plutei of Echinus miliaris. 

 By HmosHi Ohshima, Kyushiu Imperial University, Fukuoka, Japan. 



(Communicated by Prof. E. W. MacBride, F.E.S. Received February 15, 1921.) 



"While engaged in artificial rearing of Echinus miliaris under the guidance 

 of Prof. MacBride,* I have come across a number of abnormal plutei which 

 had the hydroccele developed on the right side instead of in its normal position 

 on the left side. Such reversed larvae were first found on May 31, 1920, 

 when they were eleven days old (text-fig. 1). The " larval " body was quite 

 normal both in size and shape, but the hydroccele, stone-canal, axial sinus, 

 madreporic vesicle, and amniotic invagination were all situated on the right 

 side, so that the larva became a perfect mirror-image of the normal larva. 

 Such larvas developed further with exactly the same rate of growth as the 

 normal ones, an echinus-rudiment being well developed on the right side, until 

 at last, when a month old, some few of them passed metamorphosis. The 

 young urchins (text-fig. 2) showed no features visible externally which differed 

 from those of urchins metamorphosed from normal larvae. A similar case 

 has previously been described only by Eunnstrom in two individuals found 

 among artificially reared larvae of Strongylocentrotus lividus({9), pp. 2-3, 7-10 ; 

 (12), pp. 419-24, Plate 14, figs. 12-16). In other classes of Echinoderms, 

 auriculariae with the hydroccele on the right side were noticed by Miiller 

 many years ago ((7), pp. 101, 109 ; Plate 5, fig. 1), and a similar state of 

 affairs in two plutei of Ophionotus hexactis has recently been discovered by 



* It is my pleasant duty here to express my extreme indebtedness to Prof. E. W. 

 MacBride for his kind supervision on the present woi'k, and to Sir Sidney P. Harmer 

 for permission to use the British Museum (Natural History) Library. A more 

 detailed account of this work will be published elsewhere before long. 



