Prof. S. Loven on the Structure of the Echinoidea. 297 



ferent series is recognized bj the form aad grouping of the 

 above parts in the peristome. The numbers by which the 

 arrangement of the pores can be indicated in that species 

 (namely, in series I. a-V. h^ 2, 3, 3, 4, &c. ; and in series I. b- 

 V. ttj 2, 2, 3, 4) recurs not only in generically allied species, 

 such as Toxojmeustes hrevispinosus and T. lividus^ but also in 

 Loxechinus albits. Echinus esculentus^ Lytechinus variegatus^ 

 Tripneustes ventricosus, Boletia heteropora, AmhJypneustes 

 ovmnj Temnojyieurus toreumaiicus, in fact throughout the La- 

 tistellffi, even in the Echinometrce. In the arrangement of 

 the pores round the peristome the same numbers recur in the 

 West- Indian E. lucunter^ Linn., with striking distinctness. 

 The madreporic plate is situated, as in all others, near the 

 right anterior interradium ; and the animal's antero-jjosterior 

 does not coincide, as J. Miiller thought he found, with the 

 longest diameter of the oval test; but it is oblique, as L. Agassiz 

 supposed ; for the longitudinal diameter passes through ambu- 

 lacrum I. and the corresponding interradium 3, and in its ver- 

 tical plane are situated the lines of curvature for the flexure of 

 the test. On the other hand Meter ocentr us and Colohocentrus 

 are symmetrical ; in these, moreover, the short diameter of the 

 test is its antero-posterior axis, in which direction also the 

 peristome is elongated, with the posterior sinus deepest. This 

 is the position accepted by J. Miiller as the correct one in these 

 genera ; but the position which is thus given to the madre- 

 poric plate, he regarded as an exception from that which he 

 regarded as the normal one both in Echinus and Cidaris, near 

 the left posterior interradium. This is not the case. Except 

 in the Clypeastridte the connexion of the madreporic plate 

 with the right anterior apical plate is constant in all Echi- 

 noidea. If its position is occasionally unknown, it is found 

 in the Latistellaj by the formula of the arrangement of the pores 

 in the peristomial plates, and by the antero-posterior axis of 

 the same test, and its division into a trivium and a bivium. 



The Latistellffi have ten free pore-plates in the buccal mem- 

 brane. It might be asked whether these do not become very 

 early detached from the corona, before the auricles are yet 

 developed. Careful investigations under favourable conditions 

 ought to settle this question. A small Toxopneustes dr'oba- 

 chensisj 2 millims. in diameter, has already the five pairs of 

 large plates in the buccal membrane, each pair in front of 

 an ambulacrum (fig. 9). Of these ten plates, those of series 

 I. a-V. b are the larger, but destitute of tentacular pores ; the 

 other five, of series I. J-V. a, on the contrary, are smaller, and 

 each furnished with its pore and its tentacle ; that is to say, 

 this latter series is here, as always, inferior in size, but supe- 



A7m.& Mag. N. Hist. Ser.4. Vol.:^. 22 



